a) Most adventurous student entrepreneurs don't really MUST need a founder visa, they can use OPT. I think founders of Stripe did that.
b) Most non-immigrants already working (mostly with H1 visa) needs a founder visa. To work on a startup you founded in H1b visa is hard, consider H1b visa needs some funding/salary requirement on your startup, and that is hard to get by on the early stage of a startup. Giving these people the visa may be important to the U.S. economy consider that group of budding founders are with the most experiences.
c) Maybe approval of founder visa won't be in the near future. Right now that is really tangled with other messes in the Immigration Act, which seems hopeless at the House in the stage.
d) Really +1 for founder visa. Can't afford to lose foreign entrepreneurs to Canada.