Only reason I can think of is that they want to eliminate cheating.
If someone finds an exploit he could keep withdrawing as much as he'd want.
"The maximum withdrawal you can do at one time is 5BTC.
For wins larger than this we pay out the winnings in installments according to a schedule which we agree on together."
Why pay in installments then? Sounds like they need others to lose BTC to payout... and if this is the case, do NOT play here. 5 BTC is not alot for a casino... couple dice sites and other casinos here handle that amount easily.