1) 64-bit has bigger overhead. If you have, for example, 8 GB maximum (e.g. a notebook) then you'll have the advantage of having more RAM available by using 32-bit system. If you just use a notebook for surfing, you wouldn't care. If you did something that had specific memory needs, it was the best solution before the notebooks with more RAM possible appeared.

2) Other hardware compatibility as there is still enough hardware without 64-bit drivers. But don't expect too much there: some kind of such hardware will have problem with this patch though. You'd have to try to be sure.

Now that even notebooks allow 16 GB the patch is becoming less relevant.

MS gave users no upgrade path from 32bit to 64bit Windows.

You need a clean install, something many are hesitant to do, or can't do.