For me it's the personalities. I'm some how very invested in the drivers more than the racing. Lewis is my main driver to follow but honestly, love following many others like, Charels, Lando, Russel, Bottas, Kmag, Alonso and some others. Love the back stories and the such. Yes, Drive to Survive played a part in really getting me hooked. I started watching both F1 and Drive to Survive near the same time and it escalated quickly into something I loved following.
For me it is similar to how I felt about NASCAR back in the 90s. So many drivers and personalities I loved to follow, even though it's turn left all day.
I fully admin Indy and NASCAR is better racing. At the moment I just don't know more than 1 or 2 guys in each and have no appeal towards it. Regardless of how good the racing is.
With all that being said, if enough of the driver I like dropped from F1 at the same time, I could see myself drifting from it. That's kind of what happened with NASCAR after Dale passed. I slowly stopped watching as the drivers I enjoyed the most left.