There's no need to write paragraphs, there are simple reasons:
Long answer:
1. Management has a different mindset than the players. They're making shit as pay-to-win as possible, it's almost as if they don't give a singular fuck about the players and what they love to see, what they love to play and what they truly enjoy.
2. As you said, good game-modes were removed and not-so-nice ones were replaced. Practice was an awesome game-mode, it was getting nice players every now and then. Removed it and what did they add? Gens ( or one-block if I'm not mistaken ). Same thing goes for game-modes like sky-wars.
3. All the game-modes, whether old or new, have a lot of potential. Amazing ideas, coming from respectable people. However, the whole pay-to-win environment ruins it all for most players.
4. Lack of skills in developers. This is not even a debate. It's obvious at this point that developers lack one of two things, either skills, or time. Every time a season is dropped, we expect tens of bugs, to the point where it'd be a surprise if a season was actually dropped with less than five noticeable bugs.
5. Of course, part of what Saturxation said was indeed true. Many of the old players have grown up and now they have a life. They can't spend 24 hours a day playing video games. After all this is a 9-10 year old server. If the X player was 15 years old when he first joined, he might be 24 now. If the Y player was 20 when he joined, he might have a wife and kid now.
6. Lastly, the staff team. The staff team is filled with people who don't even care about the limits to their powers and how they're supposed to use it in a certain way. All it requires is one sentence. As soon as a friend asks something from a moderator, it's there for him. No matter how stupid and unfair it is to do it.
Short answer:
Money-thirsty owners, skill-less/busy developers, and abusers in the staff team.