Last I looked, they had ~30% of users running on 64-bit incapable hardware, and they don't want to ask techincal hardware questions on the download page. For example Intel Atom CPUs only got 64-bit relatively recently, the earlier netbook generations were 32-bit only.

Edit: found the source. It was actually 25%, a year ago. And the other reason was that the error message when trying 64-bit on incapable hardware was very cryptic. https://lists.ubuntu.com/archives/ubuntu-devel/2012-April/03...