After several game days the mob cap will be filled up by Drowned as they will spawn in 2 or 3 deep water near to surface and even during the day as light levels are lower than 7, as they are underwater they will not die due to sunlight, also if the pools have any sort of covering then you will get them spawning as cave spawns too so its not unusual to find a small pool of 16 or even more Drowned and no monsters anywhere else in the local area. This is especially bad in oceans and swamps that have a lot of water. I have noticed Slimes are rarely spawning in swamps now as the Drowned are filling up the density cap by spawning in nearby water, making it very frustrating to try get smileballs.