It's pretty stale at the moment. Every pack has the same or similar mods, mod authors are constantly having to update their mods due to increased speed MC releases updates, and there's no new interesting mods. They're just the same mods from before, just remade a bit different. FTB just recently added all the CoFH team mods to their 1.10 packs as they're now updated to 1.10, they're still pretty similar to the 1.7.10 versions, and not new enough for anyone to get excited about them. There are a few new mods that could be interesting, but are never featured in any packs. The other thing I think lead to the this state of modded minecraft is CurseForge. CurseForge/"Twitch Client" is the true curse of the modded server community. If you want an FTB pack, you have to get it on curse or try to find it on the legacy launcher. The "Twitch" client also does not support linux, so there goes another whole group of users who cannot use the client. Curse/Twitch also has many problems with the client itself, not just the Minecraft part of it. It's constantly trying to be something it's not, a chat client, a video chat client, a voice client, a public chat client(like public discords), and a game launcher. It fails at all those things. The Minecraft part of the launcher is pretty bad, it fails to install everything sometimes, resulting in packs being unable to launch. It says it focuses on performance, but
And that's just opening it and leaving it on the home screen. Twitch/Curse has also lead to pack quality degrading, as modpack authors can just scroll through a menu and pick a bunch of mods to throw into a pack. I also think many of the popular Minecraft youtubers don't promote packs as much as they have before and have moved on to "Lucky Block Challenges" or something of the sort. Youtuber packs still gain the most players, as players want to play the same pack as their favorite youtubers do. I currently run a TrollCraft server(Modpack by Kehaan and played by a bunch of youtubers), and it gets around 30+ players at peak times.
tl;dr Few new mods that aren't in any packs, Twitch is a Curse, modpack authors are getting lazy, players want youtuber packs.