Speaking as someone who has gotten addicted to online chess in the past couple years, I think it's a combination of factors:

- Short time format makes for intense, fast paced games

- Starting a game only takes a few taps, and you can be done in a few minutes

- ELO ratings give you a number to obsess over improving, and online matching puts you against even opponents

- Instant post-game analysis

- Large amount of Twitch/Youtube content make it more fun to learn and follow the community