Basically, pools are harder to "monitor" after dark, and the danger level escalates.
First, it costs more to hire lifeguards to work in the dark, and even if some were on duty, it would require more effort (extra guards, extra lighting, etc.) to provide the same level of protection than during daylight hours. Which is to say it is more expensive. Insurance costs are also likely to be higher after dark.
Then there is the potential problem of noise. Putting aside the issue that people are likely to be more noisy after dark, what noise exists is more likely to disturb sleeping patrons during "dark," as opposed to daylight, hours.