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...

It's actually just poor wording: http://askubuntu.com/questions/1441/why-does-ubuntu-download...

A few drivers and Flash used to have issues with the 64-bit versions. Also 32-bit works everywhere, whereas 64-bit doesn't. Ubuntu tries to be n00b-friendly and recommending the 32-bit version stacks the odds in Canonical's favor that inexperienced users will have a better first impression during install and first use.

Been using 64-bit for the last 5-6 years I think, never experienced any major issues with it. Only weird isses I've had were browser plugins (java and flash), which were a bit dodgy on 64-bit, but that was years ago.