In the old days, it was a deliberate policy, designed to get guests back into the casino early in the evening.
These days, it's more about safety and economics. A greater liability is involved in overseeing a pool once it gets dark and it's not cost-effective to provide lifeguards, lighting, and cocktail service when most of hotel-casino patrons are off partying somewhere else.
That said, it still holds true that every Las Vegas resort-casino would prefer that its patrons were spending money inside the casino than lounging by the pool for free. So most pools close at sunset, around 8 p.m. in the summer and 5 p.m. or so in the winter (for those pools open year-round, of which there are many).
If you really want to go for a swim and your hotel doesn't have a pool that's open, or a heated pool if it's winter, a number of the spas have their own pool, including those at the JW Marriott in Summerlin, South Point, and Westgate, they're all open to the public. If you don't mind paying the admission fee, which ranges from about $15 up to $45 for non-hotel guests, depending on the spa, then this is another option worth bearing in mind.