Financial transactions take some time to process even deposits. Poker Rooms have agreement with payment processors to that allows deposits to made available immediately when in fact these take a few days for the Poker Room to actually receive the funds.
The wait time on withdrawal act as a security measure giving the Poker Room time to verify that there is no fraud. Examples malfunction of software or persons with stolen cards to deposit.
All Rooms also process payments in batches it makes sense to do it that way as it is cost efficient. If your withdrawal was not processed in time for current batch you have to wait until next batch for payment to be sent out this might be next day.
Some methods like wire transfers naturally take longer. If you go to a bank to do a wire transfer for anything (not poker) that has to go through 2 or 3 banks you will understand what I mean.
There are some rooms and some methods that available immediately or very fast. I will give you examples.
On
pokerstars I can deposit via
skrill or EcoPayz. They do have an automatic 48 hour wait period from when a deposit is made to when a withdrawal can be made. Once this 48 hour period is over I can withdraw to either of these methods and once I have enough deposit amount in last 12 months they run a credit back to the method. This allows for withdrawal to be made instantly and funds are back in my account and ready to use immediately.
On
party poker there is no automatic wait time. When I make a withdrawal to either Skrill or ecopayz (which are methods I used many times to deposit and withdraw before) it goes in queue they review it and pay it out I normally get funds 12-24 hours the timing is determinant by the time of day I make request and probably how many other transactions that are ahead of me in the queue.