"T-O-Y-O-T-A"
(closely associated with: "Lexus")
In other words, brand cachet.
Being a 'mid-size' (by USDOT standards, anyway), it probably appeals to the consumer as 'big, but not too big'. It's also considered more off-road capable than the Highlander, and there are parts from TRD (their performance parts division) for off-road, etc. Also cheaper than a Land Cruiser, Sequoia, or their Lexus equivalents. I'm guessing full-size SUVs are considered pariahs in CA these days.
Toyotas have a reputation for lasting forever, with a minimum of care. That's 70% of it. People who drive them are regarded as 'smart' (by people who aren't gearheads, at least). Even Priuses (Priii - ?) get that reputation - their hybrid battery pack generally lasts about 250K mi, on average.
I agree with you on the interior room (and I also agree with you on gated shifters, which Toyota loves to put into their vehicles.)
I don't know if the 4Runner platform is sold in Japan, but if it is, their vehicle registration tax is based on the size of a vehicle - so the smallest (called 'kei' cars - think of something smaller than a Yaris) are taxed least. Something like a 4Runner would be considered an 'executive level' car (taxes would be expensive.) The most popular of the 'kei' cars over there is the Honda Fit, btw. While I'd never own one, it's the inverse of the 4Runner - massive interior space for its small exterior - and it's configurable in different ways.
There's a saying about Toyotas that I like - "the least amount of car, for the most amount of money". To me, it applies. Every time I drive one (or a Lexus,) I'm disappointed by some aspect(s) of the car big enough to discourage me from buying it, and look for something else. On top of that, there's nothing that 'excites' me about Toyotas. Whoever coined the term 'soulless appliance' - it fits.
They also have a thing with their options packages - it appears that it's nigh impossible to order a car with a specific package. Rather, your dealer searches regional inventory and *hopes* there's someone in his area who has one with the package you want, *and* willing to do a dealer trade. To me, that's ridiculous. But people keep buying them, so there's no reason for Toyota to change that strategy. I've had Hondas located for me, but it's usually for color preference.
I've been to Cali, and I know what you speak of - sometimes on the interstates, it seems like every other car is a Toyota (well, maybe a 1/3, anyway.) Toyotas are fairly popular in Chicago metro as well - but I think the past few years, Honda has overtaken them around here as the most popular Asian marque - and they, in turn have been overtaken by Hyundai / Kia (which, IMO, are NOT as refined as Honda or Toyota. That being said, I think Honda especially has slipped in the last few years, and that's helping people make the switch to Hyundai as well (not to mention price). And they'll be overtaken by the Chinese, someday.
Still another reason for Toyota's success - immigration. Like it or not, Toyota is a world brand. Their pickups are the stuff of legend for taking a beating. So - when people from other countries establish residence here (Middle East, Africa, Asia, India, Pakistan, etc.) - they start with a used Corolla, and many times, stay loyal to Toyota as they become more upwardly mobile. In a way, I understand that - they go with what they're familiar with.
In short - no one gets ridiculed by buying a Toyota.
Do I agree with that assessment? No. But I'm more involved with automobiles than the average person (not as much as I used to be, but still.)
I don't drive a SUV to be considered 'smart' or 'upwardly mobile'. I drive one because it fits what I need it for better than any other vehicle.
I drive a GMT370 because it has the room / power / capability I need, it will last next to forever with proper care, I can generally fix anything wrong with it, and parts are considerably cheaper than with most import brands.