You may not have noticed it but Minecraft modding has been in a very bad state, especially since 1.8, mostly thanks to Mojang and Microsoft. Updating to anything past Minecraft 1.7.10 may not be possible for quite a while, possibly never, same goes to almost every other medium/large modpack.
Mojang had been promising a modding API since 2011, a system that could replace Forge (the fan made mod API) and take away the huge amount of work that both the Forge team and the modders spent updating to the next Minecraft version. They have failed to make any progress on that mod API at all, in fact they’ve failed in almost every effort to support modding despite benefiting from it massively.
Straw that broke the camels back: 1.8
With all of those problems and more in mind it was only a matter of time before something went even more wrong, that something was the 1.8 update. The 1.8 update introduced massive backend changes to the Minecraft, changes that meant any modder updating from the previous 1.7.10 version had an even more massive amount of work to do.
At this point most modders just gave up. It could have been avoided if Mojang finished the mod API. It could have been avoided if Mojang communicated better with their modding community. It could have been avoided if they adapted the update to hurt modders less. But none of those things happened.
Going off MCF Modlist statistics the number of available mods has dropped from 1239 for version 1.7.10 to 431 for version 1.8 then to an even more disappointing 23 for version 1.9. Most of those 431 not even being the most popular mods.
If we updated to 1.9 we’d lose almost all of our mods, the weather, the mobs, the tools, the terrain generation, etc, etc and would gain nearly nothing. Even updating to 1.8 would have us lose quite a bit for very little benefit.
TL;DR: Mojang is bad at supporting modders and as a result both 1.8 and 1.9 have barely any mods, not really enough to make a modpack like ours (see graph).