I haven't looked into this at all but as a half-assed guess, if you went to download the Desktop version it's because while you can run the 32bit version on 64bit hardware the reverse is not true and most people don't know what the heck they have, so newbies who just go and download the default will be able to run the OS that they download. If you know enough to know that a)you've got the hardware and b)you know you want to run it, then you'll look for the 64bit version.

I don't have 64bit hardware, and a lot of people who go to Ubuntu do so because it can run better than other OSs on older less capable hardware, I don't think it's a totally stupid move on their part. Especially since some people just see that 64 is bigger than 32 and think that it will be better/faster not knowing what it means.