The biggest reason is that it's a reaction to RF-ification. Starting back on 1.7.10, there was a big push for every mod to move over to RF. This eventually cumulated in RF being renamed FE and becoming part of the Forge API. This was good for cross-mod compatibility and came to dominate the space.
1.12.2 was peak RF. Before Create, you'd have to go all the way back to Rotarycraft or Electrical Age on 1.7.10 to find a tech mod that wasn't seeped in RF, and even then people sometimes used these mods solely for generating RF. The closest you could get on 1.12.2 was Better With Mods, the Better than Wolves spin-off. (To play the original Better than Wolves, you have to go all the way back to 1.5.2! It still receives updates, but I'm getting off track.) True, there was Industrialcraft, which used EU, but 1.12.2 saw it get displaced by Immersive Engineering, which was somewhat similar in function but used RF!
The problem with RF is that it's kinda boring. You make it, you move it, you store it, and you burn it. It's fine. It functions, but the power transfer isn't that interesting. You also had "magic blocks/multiblocks," which were powerful machines that were basically plug and play and didn't need much design to get working right.
So when Create showed up, it wiped all that away and used rotary power. No RF, no magic blocks. Here are the pieces, figure it out. Nothing like it had existed for several versions. On top of that, it had a vanilla-plus vibe, which people like. It's visually interesting, which people also like. Stuff like that gets r/feedthebeast's attention, which gets the packdevs and the YouTubers attention, which leads to modpacks and videos, which leads to attention over the whole modded Minecraft scene.
On top of that, it is a genuinely good mod. It is in everything, though, kind of like how Tinker's Construct was in everything for awhile. Eventually people burn out on it due to overexposure and the pendulum swings back. How many packs have Tinker's now? Plenty, I'm sure, but not like it once was.