For most people, they can already see the internet in Chrome, or Safari, or maybe even Internet Explorer. Why would they switch when they don't even know what they're switching to or why?
Meanwhile Chrome has the full force of Google behind it, and Chrome is actually one of their important projects that they aren't going to abandon any time soon. It's really Chrome vs. the world, and Chrome is winning.
Why would more users make the effort in 2018 than in 2019? This doesn't explain the trend though it's probably an accurate assessment.
I'm firmly in the latter camp, and if it weren't for the security implications I'd still be running pre-57 Firefox with the full set of extensions I found useful. If I didn't need to use all the major browsers anyway because of my web development work and I wasn't so untrusting of Google in terms of privacy, I might easily have decided at that point that Firefox no longer had any compelling advantage over Chrome and switched. Presumably some people did.