A lot of small issues that added up to an overall negative experience. The web UI redesign a few years ago that made it the same as the UI in Android, smart TVs, etc, but also slower and more difficult to use. Autoplaying something if you hover over it for more than a second (a cancerous UX design that has spread beyond Netflix now). UI inconsistencies like the "resume watching" row. 50% of the time it would be at the top where you would expect it, 25% of the time it would show up if you scrolled far enough, and 25% of the time it would never show up at all. I never understood that one, it was bizarre and unexpected behavior. Removing a lot of quality content and not replacing it equally high quality content. The overall decline in quality of Netflix's original series.
There wasn't any one thing that made me decide to cancel, one day I realized I hadn't used it in several months and the UI made it a pain to use when I did, so I canceled.