Because subscriptions are only part of the revenue stream. The other part of the revenue stream is getting eyeballs on and participation in sponsored challenges, and other sponsored things. The more people who sign up for the Rapha Rising challenge or whatever, the higher the rate Strava can demand from sponsors like Rapha. If people start drifting off into Garmin Connect, Trainerroad, Endomondo, etc, it's a problem.

I think in Strava's perfect world they'd be free for the end-user and would just get big checks from all the equipment manufacturers to get access to the Strava user base.

But that perfect world doesn't seem to be working out.

But Strava can't just cater only to the paying subscribers. They need to keep everyone coming back.