If you are indeed talking about large withdrawals the reason could be their cold storrage. Any reasonable big amount of coins should not be storred online. A securitiy machanism for this storrage could make withdrawals only possible every 12 hours (e.g. need confirmationd of at least 2 out of 3 possible people)
If they dont use cold storage for majority of there funds I wouldn't be using them at all. This is exactly why some larger withdraws take time because they're manually processed.