The issue is minecraft has become oversaturated.
To put it simply; Minecraft is old.
It's out of its prime.
So people invent mods, and Forge (and now this new Cauldron, which I'll delve into later) spit out APIs so you can put mods into your server. Vanilla minecraft, simply, isn't fun anymore. We know what to do, how to play it, how to (in theory) beat it. With all of these new mods coming out, seemingly daily, it's impossible for Mojang to keep up with the demand for new content. Thousands upon thousands of modders can do what Mojang wishes to do in months, in days.
And now with these modpacks that make mod installation as simple as ever, more and more of the less tech savvy Minecraft community is starting to turn to mods.
Before things like FTB and Technic you had to manually install mods. This made it difficult for modded servers to grow big with multiple mods (eg; pixelmon servers were pretty big, but it was pixelmon and pixelmon only. You didn't get pixelmon servers with other mods. It just didn't happen.) because nobody was going to take the time to manually install 100+ mods for a server that may, or may not, be anything worth playing. But now that you can install 240+ mods (I'm looking at you Unleashed) with the simple click of a button, it's insanely easy for these modded servers (which have easily 100x the content of the original vanilla servers) to grow way more populated than they could have dreamed of in the past.
And now not only do things like FTB and Technic exist; they're popular. They've seen growth in the hundreds of percents (300%, 400%) in the past few years, simply because; Minecraft is old. It's boring. It's lacking.
Wow, Minecraft added a new biome.
Biomes O'plenty adds, what, 15?
Wow, Minecraft added a new redstone feature.
There's mods that have added thousands of redstone features that have been around for years. Literally.
Minecraft added new animals or food?
Mo'Creatures and and HarvestCraft fix those issues amazingly.
That's just scratching the surface of the, literally, thousands and thousands and thousands of mods that are available to players with simple buttons.
