Stu, I think that question is difficult to answer without sounding sanctimonious and self-righteous ("I've seen the light, unlike those cage-dwelling troglodytes!"), but I guess I'd say that people don't get it because they haven't tried it, and if they don't try it, they'll never get it. (And I have to be fair and mention that there are some people who are simply unable to be car-free or car-lite because of their health or their job or their living situation or their lack of access to public transportation.)
There's a class element to it: it's cool to zoom around in your car. It's not cool to ride the bus. Riding your bike is for poor people or eccentrics. I've heard people say that they refuse to ride the bus because they don't want to be in such close proximity to "undesirable" (read: poor) people.
A more charitable interpretation is that it IS unpleasant to be crammed into a crowded bus like sardines along with a teeming mass of humanity. In inclement weather, it is unpleasant to wait at a bus stop or walk a long distance or ride your bike. It can be inconvenient to spend a long time waiting for the bus.
It can be intimidating to try something new. Novice bus riders can have difficulty finding the correct route or deciphering schedules. The first several times I rode a PAT bus, I was so embarrassed and unsure because I just couldn't get the hang of whether you were supposed to pay as you were getting on the bus or as you were leaving. (Actually, to this day, I still don't really know. There's supposed to be some rhyme or reason to it, but I can never seem to remember it. Now I just forge on, regardless of fare-paying humiliation.)
Starting to ride a bike has its own embarrassments and difficulties, too. When I first started cycling, I was absolutely incredulous that you were supposed to ride a bicycle in the street and not on the sidewalk. I was in awe of the hot hipster dudes (hi, hot hipster dudes!) cycling briskly down the street, and I thought that I could never, never do that in a million years.
I didn't have a car in Pittsburgh as an undergrad. I also hadn't been turned on to the wonders of riding a bicycle, and I did feel severely limited because I had to rely on the bus and my own two feet for transportation. Relying solely on the bus can be inconvenient: it may not run often enough; it doesn't run late enough at night; it doesn't go exactly where you want to go so you have to walk a long while at either end; you might have to transfer repeatedly...the list goes on. I really think multi-modal transportation helps a lot in making up the gaps in Pittsburgh's public transportation system.
I also think people don't realize how much riding a bike broadens your horizons. There are places in Pittsburgh where I will not drive because it's such a massive pain in the ass to take a car there. Getting there by bus or bike is infinitely easier, and it means you don't have to look for parking (or pay for it!).
As for how we can help encourage transportation alternatives--well, I think rising gas prices will help with that. So will activism, both active and passive (riding your bike and showing people by example that it is a fun, convenient, viable, and realistic form of transportation).
Well, that's my treatise, but I think I'm probably preaching to the choir here. 
