Official subs getting better is a big part of it. The gains for a fansubbed release are now mostly down to better encodes/actual TSing/song translations. Putting in all that effort for a fraction of the viewership on a good day isn't that appealing to many people, especially when you add in the delays/workload associated with doing that work to a high standard (For example, many people have seen how long it takes GJM to do the studio-quality TS work it does on many shows and the release cadence for their projects as a result of that).

As a more nuanced answer: Far fewer people pick up fansubbing as a hobby now since most seasonal anime is covered by official releases and doesn't require people to step up to the plate to fill in the licensing gaps. A lot of those who may have decided to fansub 10 years ago would instead today choose to scanlate most likely. The demand is just much greater there. On top of that, most fansubbers now are much older and don't have the free time to devote to blasting out episodes of (multiple?) shows as they air at bumfuck'o'clock in the middle of the week as many of us old folks did as college kids in the early 2010s.

There'll always be a niche for archival-quality work from fansubs—I doubt that'll ever go away. But fansubbing as the way to watch anime as it airs is practically dead with a few exceptions every few seasons, those usually being thanks to Netflix. As mentioned before, the market's saturated by official releases and more of them are getting better on average as time goes on. As for whether many of the good official translators of those shows stay in the field is another question though, since at least at Crunchyroll, their pay is atrociously shit compared to the industry average. This is problematic as Crunchyroll usually covers the licensing acquisitions for the majority of an airing season, so if they bleed out their good translators, things may slip backward in terms of the quality of their final products. Luckily, Sentai licenses are usually high-quality jobs, at least, and their staff are paid much more sensibly, but they grab far fewer shows in comparison.

That should cover a lot of points, I suppose. Hope you were expecting a wall of text.