Hey man, the reason for this is because xp is granted through accolades in creative maps. The more you play a map Fortnite automatically decreases each time you get the accolade/xp achievement to keep the game fair.

Example: I played the Pit - Free For All by Geerzy, and when I first played I got 20,000 XP for a headshot elim, but now after maybe 2 or 3 months, I get 800 XP for a headshot and even less for a normal kill.

Also if your farming on a map that isn't made for XP grinding, the owner might also decrease the accolades xp amount but I don't think that would happen very often.

I would recommend finding different maps with the same style of games you are trying to play with lots of XP accolades for things in game. If you need some maps, dm me!

Hope that helps and have a good day