With coin slots, the casino has to keep them stocked with coins, a quarter machine probably takes a few hundred dollars worth. This is non-working capitol. The big casinos have thousands of slots. Would you rather have a $1,000,000 in the bank earning interest (or put to some other productive use) or just sitting around in slot machines?

I'll also bet that coinless slots are a lot more reliable than slots that accept coins because of the reduced number of mechanical parts.

Personally, I would much rather have the slots with vouchers. After an hour or so of handling coins or tokens, my hands are filthy.