I can't explain why someone might prefer it over another battle royale game (or COD), but within the Overwatch community there's a sense that Apex gets a lot of things "right" that Overwatch is struggling with. Almost all of my friends have switched to Apex specifically because they find it less frustrating than Overwatch.
1) Team communication is easy, even without mics. The ping system is extremely easy to use and surprisingly flexible.
2) Even though you're on a team, your individual contribution feels like it matters. In OW the rest of the team's composition can screw you, and it's frustrating to know you're playing your best but it doesn't matter because your team comp is messed up.
3) Games are quick and losing doesn't feel as personal. So what if you died in the first two minutes, you can quickly hop into another game.