A couple ideas pop to mind...
1. The rest have been out a few years longer.
The latest Sentra just came out. And competing with the prior Sentra model didn't make life good for Nissan.
In test ratings, the car magazines have picked the others above the prior Sentra model, thus giving it few chances here.
"The Mentality-being, what?! Yo stupid? Don't know better than to buy the best rated cars?!"
With the huge number of freeway miles people cover here, it becomes an important thing to pick a better car (both in ratings and reliability) or you'll get the harsh stare of fellow workers wondering why you're late again because you're car broke down.
2. Better marketing.
It could be that people like the ads (print and video) from Honda, Ford, and Toyota more.
Also, Nissan has never managed to associate the Sentra with more than just a 'cheap buy' in the minds of many potential buyers, whereas Honda, Ford, and Toyota have managed to 'upscale' their cars successfully. Certainly, even buying a Honda Civic here in California acquires you a certain 'ooh, nice' that you don't get with the cheaper cars, like the Hyundai's.
The compact tuner magazines have also contributed towards this image by tossing cuties on Hondas and Fords, but few Nissans on their front covers - not to mention there are much more parts for Hondas than Nissans so their focus is naturally thus biased.
also, an ailing parent company (Nissan) means fewer ad dollars = fewer sales = crumbling market share. a nice, tidy sales blackhole they're desperately trying to climb out of right now (and not just here! Same is true in home town Japan as well.)
3. Better styling.
It could be that people like the styling of the other cars more. eg. the big-eyed Civic headlights are well-known in the design industry as one of the Civic's strongest selling points.
You see a Ford or Honda and it is distinctive. See a Nissan (esp. prior model) and it could well disappear in the eyes of the younger buyers here.
Go further into head/tail design, and you'll see that the 'faces' they look like will affect whether the buyers 'like' these cars more or less.
(again, those big eyes up front on Civics sell those babys - take them away and they'd never sell as well)
4. More performance upgrade parts for the Hondas.
5. Better feel/ergonomics.
Subjective, but lots of people love that 'Toyota' feel from everything about the feel of the plastic dash to the silky click of the switches to the feel of the seat and floor fabrics and will pick their next car based on that alone.
Nissans tend to vary in their feel, and they don't seem as 'rich/good/nice/comfy/durable' as the Toyota/Honda feel to some people.
It may be that better ergonomics does it for some as well. Maybe a seat that is just a bit higher so shorter chicks can see out easier. Maybe it's they don't put the trunk release up front when everyone expects it down there. Maybe it's because the outside mirror controls are easier to use than the pad-on-a-stalk thing in the latest Sentra.
6. Better roadfeel.
Double-wishbone suspension in the Hondas coupled with low hood = less of a cramped, can't see the road feeling. Maybe people buy a lot based on that as well as the better handling.
7. Better suspension feedback and quiteness.
The Corolla probably gets a lot of people on the fact that they're well controlled over the usual freeway/road bumps, and decently quiet handling such. That is before the latest Sentra arrived...
8. Everyone else has one.
People follow the pack. With Toyota & Honda already entrenched, friends buy what their friends drive.
9. Very good service and reliability.
Dealership service and high reliability ratings push Toyota and Honda ahead. People swear by how long they last and run w/o trouble or breakdown. With Nissan's, it's touch and go. (Of course, the Honda's don't have the metal drive belt the Nissan's have, so they get stranded by the roadside when those break 
While some of this is imaginary word-of-mouth, it does a lot to paint a rosier picture in their minds.
10. Their parents don't drive Nissans.
More people drive Toyotas, Hondas & Fords than Nissans (same goes with sales ranks here, too).
Mommies and Daddies probably already drive a Camry, Accord, or Taurus (how many Maximas do you see vs. those?), and they are more likely to return to their dealers to buy up a Toyota, Honda, Ford for their kids because of their past experiences with their own cars.
With the high resale value of Hondas, extremely good reliability of Camrys, and just widespread sales of Taurus', these all provide them with reasons to buy the same brand for their kids.
11. Much higher resale values for Hondas.
12. No visible people in their Sentra ads.
People connect with people driving cars, not just the cars alone. that's why the Volkswagen ads are great - they showcase the 'adventures' people have in/with their VWs. Not just a solitary Sentra spinning around gleamin in the light.
If nothing else, just toss in Britney or NSync and watch sales jump.
13. In the end, Nissan needs to realize it should first sell to the heart before it can sell a lot.
In its current ads, it doesn't sell much more than 'our car is roomier and bigger'. (Yes, it puts in that it isn't Republican, but who in their right minds would try politics to sell to the <30, mostly non voting, primary buyer age group?!)
Not safer. Not cuter. Not more reliable than the competition.
Nothing there to really reassure the buyers their getting a great car, or that it's really sexy. Only thing you get is a just a great buy - ie. a lot of car for the money - which is something that won't sell many cars here in California, where a crappy car will kill your image (other drivers go 'yawn') and chances at a girlfriend (thinks 'cheapskate!!'), not to mention make you target of jokes of your friends (yo! didn't have enough $$ to buy a 'real' car, eh? Now just where are you gonna get parts to upgrade it now?).
And certainly nothing like the 'oooooh, nice!' image you get when you've just bought a nice new BMW 3 series and go cruising.
(We're talking about the <30 crowd here that'll mostly buy these subcompacts for their first ride that's the primary selling target market for these makers.)
Also, what's up with mentioning Jerry at all? You don't wanna associate a <30 crowd with a >50 year ol' designer. He's got zip/nothing/nada to do with selling cars (and we sure dont' wanna 'associate' ourselves with him/his age group/beliefs/etc/.), and should be tossed altogether. (damn, if it's just a face they want, just toss in a cute model instead)
It's lifestyle that we wanna see, not an aging designer.
Not just that, the music they play isn't edgy/new enough for a lot of the younger buyers (like who listens to that old'PopDaddy music?!). Something harsher and newer, esp.for the California crowd.
If nothing else, at least showcase the SE model ripping rubber past yo all on screen! If the music ain't cutting it, at least a few crazy hairpins and 360s doing something Leathal Weapon-ish will do just fine at selling mo cars.
d 
