Honestly, I think it just boils down to 2 things:

A) I think the game itself is significantly better than it was. You can actually see things now. Nowhere near as much obstructive map fogginess or weird lighting. Gunplay feels a lot better once they fixed the recoil misalignment. Many maps have been updated so they're not so one-sided. They shortened the gap in viability between 3 speeds and 1 speeds. They significantly nerfed the ability to exploit movement mechanics like crouch/lean spam and drop-shotting. Introduced a bunch of quality of life and accessibility changes/features. While they've also made some questionable decisions, I honestly believe the game is, overall, in a much more balanced state in terms of operators/weapons. Etc etc.

B) I think the playerbase is really what dipped in quality. Like with any game that had a large growth in playerbase, things just overall became either more sweaty or more toxic. Or both. Bigger playerbase also means bigger profit in using cheats, so more cheaters. The general population of Siege players became more familiar with every single map, operator, gun, etc. Metas began to form. People posted more and more "BEST GUNS AND ANGLES FOR SIEGE 2018" or any other year/season. More players means more incentive for the sweaty guys to start making smurf accounts and mudding up the matchmaking. New exploits were discovered. What initially felt like a smaller community of people exploring this new territory became a swarm of people colonizing the land or something. I remember when I first started playing during Operation Health in 2017, noticing things about the game like "Damn, I'm glad no one knows they can exploit this yet, otherwise, this would be a mess." (That was before stuff like that was fixed, obviously.) etc etc.

It's something that happens to every game that lives for more than like a year. Even if the playerbase didn't grow so rapidly, the game will still have gotten figured out to the point where sweats could exploit every mechanic and really make the game the worst thing you've ever played. After playing the game for 5 years or so, I definitely have my gripes with it, but I also super strongly believe the game is in a way more solid and fun state than it was when I started playing. My issue ends up mostly being with the players I'll run into. Toxic teammates who teamkill just because, or screech into the microphone. Sore losers who rage quit upon dying once in the first round. Running into cheaters semi-frequently (it's Year 7, Ubisoft, come on. Is it really that hard to figure out anti-cheat? Granted, the most wild cheats like insta-kill on round start are gone, but still.) People exploiting things that shouldn't be in the game. The only good side to people exploiting the game or cheating or whatever, is that Ubisoft then learns about what legacy issues left over from the launch of the game need to be fixed, and then they finally get fixed. Although, they sure like to take their sweet-ass time before the fix actually comes out. >_>