Most people just walk into a store and get a phone that they think is cool. The Nexus One was not available anywhere except Google's website.
When I went in to the T-Mobile store, I came out with a G1 because the T-Mobile lady was explaining all of the hoops one must jump through to use a Nexus One. They don't carry them so you have to order online and wait 4-7 days for the phone to arrive (my old phone was broken, so this alone made it not an option for me), you can't use T-Mobile's normal plans on them, I wouldn't have been able to buy with subsidy because my previous contract had one month left and therefore manager override was necessary to grant the discount on the phone, and I didn't have $500-$600 cash sitting around to spend on a phone, and so on.
If the N1 got the same kind of carrier marketing and buy-in that the other phones got, I'm sure it would have done well. Carriers, however, did not want to provide that marketing or support because doing so would minimize their death grip on the end-user and count as a success for something not loaded with carrier crapware, which they don't want either.
The carriers know, of course, that nobody really _likes_ having a permanent, unremovable Blockbuster app on their phone. But since people will tolerate it, and the carrier gets a lot of money from Blockbuster (or NASCAR or whatever) for doing that, they want to keep phones that are free from this crapware as far away as possible; if they didn't, people would prefer phones that are without crapware, and this would impede the carrier's ability to sell contracts to include crapware.