That's one of the definitions of critical mass. Reputation is built up over time and usage. Gitlab's uptime reliability isn't particularly known or unknown, mostly because, compared to Github, it's not used enough for those issues to come up.
Try talking to a dev, ask them why they use Github instead of Gitlab. Are they going to tell you "Oh I absolutely would use Gitlab but their uptime is such a problem"? Or are they going to tell you their stuff's already on Github? Their friends/coworkers are on Github? Their employers look for their Github profile?
As a sidenote I would totally use MySpace instead of Facebook if not for their notable history of extended downtime...