Because the most matured clients for the most part are there (at least damus). They are also native apps so they are much faster than any web client.
Discussion
Recommend and good desktop Linux apps ?
The first two that come to mind are https://github.com/mikedilger/gossip which is rust based and https://github.com/unclebob/more-speech which is written in Clojure. Gossip is probably easier as I have never used more-speech