The problem is that to direct a login, we have to tie a specific value to a specific URL. If you have a multi-select option like checkboxes, that can't work. The code becomes too complex. If I check 'Music' and 'Arts' as my interests, then how what page do I get when I log in? You would end up having to go through permutations to set up pages for various combinations.
That is way too complicated for any normal usage.
The system is designed for a single select item. You pick each value and assign each value to a URL. After all, a user can only login once and will only see one page after logging in.
Regards,
Glen