Buying on one exchange then selling on the other I'm curious as to what is keeping the majority of the people from buying on one exchange, where the going USD/BTC rare is cheaper, and selling it on another, where the rate can be USD$50 to USD$100 higher.

There are probably people doing that already. But since opening up accounts in multiple exchanges is no feat, why isn't a much larger portion of active traders doing this?

PS: I understand the answer MAY be speculative, thus not suitable for SE. At the same time, the answer might NOT be speculative - there might be a solid reason behind it. So I'm here to learn. Please vote for closure if you see fit, but do not downvote, given that I am aware of what kind of questions should be asked here.
Well for one, it's almost impossible to get money off of Mt.Gox. (I had a wire transfer pending for 3 months and finally decided to just cancel it and take the loss.) Some bitcoin exchanges refuse to do business with the US (btc-e) and some only do business with the US (coinbase). Bank transfers are not cheep and the exchanges take a somewhat large fee for moving money onto and off of their exchange (~1% each way.) 

But some people do make money like this. You will find though, if you actually do the math for all the transfers and fees and what not that you would only be making a small margin and that only sometimes. 

It's absolutely doable but you need to be doing it at scale and with good and fast programs managing your assets to really make a profit at it.