Why does chess.com use stockfish 11?

I've noticed that in chess.com only it uses stockfish 11 but sites like lichess, use stockfish 15 NNUE.

Any explaination?

Stockfish 15 was available for Chrome and Chromium based browsers until early January. Chrome released update 109 which caused browser crashes after games and while spectating for members that had Stockfish 15 configured as their analysis engine.

Due to those crashes, it was temporarily removed and the previous configured version was set. 

It should come back at some point when the cause of the crashing can be determined and fixed 

Alr but stockfish 11 is only browser engine the other kind is stockfish 15 why is that?

Stockfish 11 in that section is the engine that runs in the browser. The other option is what runs when doing a Max position analysis and runs on a site server process.

alr thx