It's all about the volume if I'm correct. The supply and demand in each exchange is slightly if not largely different. So more demand obviously means higher price and vice versa. They are not that significant though.
The difference in different exchanges is not so significant to be able to do arbitrage trading.
It can't be as I believe these days, there are arbitrage bots and there are those who are trying to take advantage of that arbitrage every single minute to make it worthwhile for them. As a result of this, it is usually first come first serve. Nevertheless, the market is maturing gradually and most especially for the high volume markets.
For btc, the difference is not even something that would be worth it, considering the time it would take for you to complete a single transaction, with the level at which price could change within that period. For some alts though, this could still be possible