(1) I think a lot of the adversion to bicycle commuting comes form the incredibly self-centered bunker mentality which suggests that the ultimate goal of suburban living is to aquire a Ford Expedition to better protect yourself and your family. Why else do you think so many grown adults are whining about how "expensive" it is to fuel their V8? Note, of course, that this also means the vehicle of choice is all the more deadly to those around you, not only in sheer mass but also in more limited visibility.
(2) Of course, that's only part of the story. Perhaps an even bigger part of the story is the extreme shortsightedness of the US's ridiculous zoning laws which suggest that every quart of milk requires making a trip in said Ford Expedition. Since things are so far away, more driving is done which as we all know makes things more difficult for those on bikes. It's now finally becoming clear how incredibly stupid it was to do things this way. Unfortunately things are so far gone now that we may be largely stuck with what we have for a long time.
(3) People are lazy. Let's face it, America wants to drive, and it's willing to put up with all of the pollution and environmentally destructive practices required to keep driving, including such insanities as burning our food supply (corn).
So, once you consider (2) and (3), item (1) is all the more easily justifiable. Fortunately (and unfortunately), I believe we're now starting to run into the real natural limits that almost everyone has pretended don't exist. Hopefully we'll finally realize that we should start to do things better, unfortunately (4) the vast majority of people don't seem to be capable or willing to consider the future. I hope that we'll start to behave that way less, and that we bicycle commuters will eventually be seen as the early adopters of a more sustainable and responsible lifestyle. Time to start doing things that we should be doing instead of the things that we can do.