I think there are several reasons for this.
1. Introduction of Minecraft on many more platforms - this has basically meant that most of the new sales are on console and therefore the bedrock edition, less people come through into the java community
2. Enforcement of the EULA for servers - Servers like myself who have been around for 6 years struggle to pay the bills. This means that nobody opens up new servers and therefore nobody invests in development for new gamemodes and ideas which the YouTubers would use for new content. Essentially because of how hard it is to run a successful EULA compliant server without thousands of players online very few new ones start.
Years ago loads would create their own server and community constantly bringing in new ideas and competition. These days most of the larger servers are all owned by the same people and therefore, there is very limited competition for new ideas.
3. Boring updates - Not much interesting has been added to the game in a very long time. The aqua update looks like the first major update that has a serious content addition. The development rate seriously slowed down to the extent that apart from the last couple of updates there was a period were there was only one major update a year.
