I think one of the bigger reasons that the modding scene has dwindled is due to competing versions of the game. Bedrock didn’t even support mods until a few years ago, and even then it was in a very limited capacity. As such, a whole generation became exposed to and grew up with Bedrock being what they played. Meanwhile, the playerbase for Java Edition slowly lost its playerbase and subsequently their modding community.

That’s just how I’ve viewed it, anyway. I could be wrong, it’s been several years since I last played in earnest. I’m sure the game simply just getting older and more modern games replicating–and in some cases even improving–many of its features has also contributed to its decline. Regardless, it’s my understanding that the game is still massively popular.