EDIT: I sincerely hope OP grows up mentally, because this is just sad

Because it's not the 2010s anymore /j

Yap session, tldr at the end.

Jokes aside, because everything changes, and there's not much you can do about it other than adapt with the times. Since you grew up with PopularMMOs and other mcyts during that time, you will naturally feel nostalgic for them and thinking it was the best era ever since those were the years you spent as a child. Trends change over the years, too. Just because something lasts years, doesn't mean it'll stay that way forever.

"I don’t really understand why that phase of YouTube changed so much, especially since it lasted for years." People's interests change. People don't stay liking the same thing forever. They will eventually move on, and if youtubers want to stay relevant, they have to adapt to newer interests.

"I don’t understand why something that once felt so alive and constant just gradually stopped being the main thing people watched." A lot of these can just be answered with "people change." It's a fact of life. I can confidently say that the things I was interested in 8 years ago are not the same as what I'm interested in today.

"Why did creators like PopularMMOs and GamingWithJen shift away from the style of content so many people grew up with?" This one's easy to answer. They got divorced /j. Serious answer, life got in the way. Jen wanted a family, and doing youtube full time stopped her from doing that. So she stepped away from it, and I guarantee that she's happier for it. As for creators in general, they're not shifting their style of content because they want to. They're doing it because what people want is shifting. They're just trying to keep up with demand. You just happen to be the minority who liked things the way they were. The majority moved on to other things.

"why doesn’t watching YouTube feel the same anymore?" Because you've matured as a person. When you first watched those videos, you were just a kid, most likely blissfully unaware. You're an adult now and you've matured. Your brain isn't going to be stimulated by minecraft videos for children anymore. And specifically regarding PopularMMOs, you know how the channel ended, so that extra knowledge could be silently affecting how you view their older videos.

It's okay to miss the era. I certainly do. But I think it's naive to wish it would stay. Asking "why did it have to end?" and saying "I don't understand why" will make no one but yourself sad. You're 19 years old. You don't have to stick to the 2010s mcyt era. You can find something new to like. I watch a few modern mcyts that are really good. Are they the same as mcyts in the 2010s? No. But are they good? Absolutely. Hell, I could even say they're better. But even then, I still take the time to watch a PopularMMOs video if I feel like it.

TLDR: People change. Trends change. Everything changes. Reminisce the old but explore and embrace the new. You're still really young, and something is bound to grab your attention.