It is the fair question and there are four honest answers.
The Gulf closes. Double red flags fly on this coast more often than visitors expect, and when they do, entering the water is a city ordinance offence in Gulf Shores rather than a judgement call. Waterville does not care what the surf is doing.
Lifeguards. A wave pool has staff every few yards watching a fixed, contained body of water. Alabama's public beach has a long shoreline and a small number of stands on it. For a family with weak swimmers, that difference is the product.
A lazy river is not a thing the ocean does. Neither are slides. Small children who find real waves frightening will happily spend six hours in a wave pool that stops every few minutes.
Sand. Some people simply do not want any more of it in the car.
Against that: it costs money, it is loud, it is busy in July, and the Gulf is free, beautiful and two minutes away. The sensible pattern is the local one. Beach most days, Waterville on the day the flags are up or the day everyone has had enough of sand.