Well, because you can't. You can't just simply walk into the US. That's the whole point why everyone raves about the Canadian Startup Visa. The requirements for the US for a foreign entrepreneur are one hell of a lot tougher in terms of funding/jobs created by company/etc.
That's an excellent question. My take on this is that by being more open to start-ups Canada hopes that if the numbers are high enough that they'll have another RIM on their hands one of these days. It costs them next to nothing (the venture capitalists / angels are picking up the tab) and they get more skilled workers either way.
I think quite a few entrepreneurs will see this as a way of moving to the states without the drawbacks (it's North America, after all) and may eventually use it as a stepping stone to the US if they're successful.
Compared to London, Berlin, Prague or Hamburg I'd stay in Europe if I had to make the choice.
If you already have a life on the go in some other place $50K is peanuts when moving to another continent. I've done it and even though we were pretty well prepared for the move the first year was astoundingly expensive. (family of 3).
For a permanent visa, requiring $10k savings is pretty reasonable. If you've been working as a programmer or engineer in the developed world, you should be able to save that much in 1 year, give or take a few months. If you're in an emergent country, 2 years or so.
My impression of this kind of visa is it is should be attempting to attract young people with high potential but without necessarily having other great routes to the life they desire in North America. If you think they are targeting a different audience, say older foreigners with established businesses/careers who just want to move to Canada then 50K is reasonable.
Also there's definite risk on one side here, otherwise it's an opportunity for no one. If you have $50K of your own, why start a company in Canada rather than SV? I think to make this a worthwhile visa, Canada has to take on the risk that they are going to let someone into their first world country with a good social safety net because that person has untapped potential. Canada takes the risk if the savings threshold is low, but if the savings threshold is high they are not creating much of an incentive to move to Canada rather than an established startup hub and its the entrepreneur taking the risk.