My opinion this is because of the difficulty in withdrawing USD and generally CASH from MtGox. Leading to continious and increased flow of cash to exchange USD-BTC since it's the only way to withdraw money from there, BitStamp withdrawals are still going (for verified accounts) and therefore the flow of USD To BTC is just the normal.
Do you have any experience in withdrawing EUR instead?
I do. I haven't raised the limit above 1000, so I made five withdrawals (SEPA) of 1000 each. The first one took 30 days before it was in my bank account. The second one still hasn't come through even though it has been ~35 days now.
Since the bitcoin price was pretty stable for the last few months Mt Gox was a good place to get more money for each coin providing you could plan in advance. Now that the price is going up again I have registered on bitstamp, I think the bitstamp-price 30 days from now will be higher than the gox-price today.