Quite a few reasons:

1/ Easy to start, hard to master.

2/ Good balance between being still athletic (you have to run), while not punish less-fit players a lot.

3/ Social component. In Portugal you must get a beer after each match. Suddenly you have 100+ friends and a shared interest.

4/ Full gradient between 'funny dumb ass game with friends with no experience' to 'professional competitive sport'. With a lot of options in between like beginners games, clubs events, amateur leagues and semi/pro tournaments.

5/ The game is more tactical, that athletic. After you get initial technical background, you start to play more 'chess' than 'overpower opponent' style.

6/ Good business. More people on less space = more revenue. More social = more spending in a bar. Coaching is more profitable as well (groups of 4).

7/ Open to all social groups. My wife is playing female-only tournaments. We play mixed tournaments together.

I play padel for 3 years, played tennis for year, tried squash and badminton.