Reddit started becoming shitty because people are shitty.
It started long before the API restrictions, long before the commercialisation.
No, it started failing when the influx of new users grew exponentially, but the systems to support it didn't.
Subreddits were no longer communities, they became information and discussion hubs.
With the influx of people, further moderation was needed. But mod vetting isn't that easy, and a lot of people who had absolutely no business moderating communities, let alone social hubs, got a whiff of power that drove them crazy.
And that leads to splinter communities, some abandoned, some forced to give up, and an overall mess to this all.