For me, the two big reasons I quit were (1) the rating system, and (2) finding a partner.
For the rating system, it’s just not competitive. Master points don’t necessarily mean anything. If you have a lot, it means you’ve played a lot - maybe they at least mean that. You could play for 50 years and easily become a life master but be complete garbage compared to the kid who did it in a month or two. There’s no information to understand how successful you actually are minus what you alone know about yourself and your game. It’s just not as fun, especially once I started to play games that make success extremely evident via stats / ranking.
Bridge is very unique in needing a partner to play with and one that you understand quite well. I once played with one of the best players in the world as a 16 year old (literally, one of the “best” at least master point wise). We did terrible lol. I didn’t know what he was doing and he couldn’t stoop down to my level all too well. How do you find the right partner? Not really sure. Again, I think it’s actually a cool part of the game, adds a unique social element and togetherness to it but it does make getting plugged into it harder.