Cost: it's cheaper to hold an event at a casino because the casino has more ancillary revenue than other venues (gambling, show tickets, room and food). Therefore, it can afford to offer the promoter a lower rent, and maybe even rebate some dollars back into the "added" fund.
Size: Pool takes up a LOT of space. Aside from casinos, you'll only find that size space in major metropolitan areas like Chicago, Los Angeles, Houston (New York is horrible; land is so expensive that none of the hotels have giant ballrooms. Your only option is the Javits Convention Center and the prices there are sky-high.) Big city = big prices.