Because they were unrelated. Bitstamp was spending its own unconfirmed change, and those spends got hung when the change was mutated by the attacker— with an attack that didn't exist while MTGox was processing withdraws. MTGox happened to never spend any coins that weren't at least 6 deep in the blockchain, even it's own change.
> and is people grasping, in the absence of anything else
It's actually MTGox's own claims. Their actual issues in this space were because they reissued payments without conflicting the original payment— allowing both to go through. No one else, as far as I can find, was performing reissues at all— much less in such an unsafe manner.
Where did they claim this? They did claim that they were suspending withdrawals due to that issue, but in actuality they had not done any withdrawals for months. It was a convenient cover at a convenient time, which they tried to extend out with the "until this problem is fixed in the protocol" nonsense.
They certainly had made withdraws within months. Here is a random address receiving payments from MTGox (the ones with the 0.001 fees) https://blockchain.info/address/1AacEsKeXqnUqtYQWKDHJV3JtpJk... if the API were still up I'd show you how to query the vin scrippubkeys to have mtgox themselves identify the txn as theirs— but it's not up now.
In both cases there was an issue with the service having transaction issues due to the malleability issue. Bitstamp suspended operations to fix the issue. How can you possibly say they are unrelated?
Now supposedly Gox was robbed (over months) using this exploit, but that simply makes no sense at all, and is people grasping, in the absence of anything else. It seems more likely that someone got the private keys on holdings wallets, whether insider or hacker.