Alternatively I am sure plenty of islands in the Caribbean would be happy to take their business (and tax revenue). For smaller players this may not be an option but Mt. Gox is a market leader and thus has more resources and more future revenue to lose.
When lots of countries outlawed online gambling the online gaming sites simply went offshore.
The whole point of establishing a presence in Europe was to offer SEPA to their European customers. There are already plenty of ways to get money in and out of the exchanges by going through third party processors but people living in Europe wanted a quick and easy way to deposit and withdraw money from the exchanges. SEPA offered that. People don't want to have to fuck around with the whole bank account > payment processor > MtGox thing to deposit and they
especially don't want to have to fuck around with the same process in reverse to withdraw.
The online gambling sites didn't just go offshore, they also routed their customers' funds through third party processors. That was the point of vulnerability when the US Department of Justice went after the online gambling sites which had been allowing US customers to play - they went after the third party processors and large amounts of player funds which were in the accounts of the payment processors were frozen.
There are already offshore exchanges - TradeHill is one - the issue is being able to offer users
quick and uncomplicated methods of depositing and withdrawing their funds without running into legal problems.