My thoughts? Because if the team gives in to every incremental request then suddenly people will ask why not Dartcraft, why not Mekanism, oh hey, why not all of UE, how about immibis' mods, what about...
There's a certain point at which a pack compiler has to make a sharp chopping motion and say "if you really want that, you're welcome to add it yourself, but it's not shipping that way". Sometimes it's to keep systems from melting under the sheer weight of the pack (there are packs out there that pride themselves on bleeding-edge and melts-anything-but-the-highest-spec systems; FTB packs tend to try to appeal more broadly, though). Sometimes the download file size is getting gigantic (thank heavens Launcher v2 is coming to help with that, but it's still a very real issue with 100MB downloads for even a couple minor tweaks at the moment). Sometimes the team doesn't want to be responsible for the extreme number of possible interaction issues that one additional mod can represent (see
this tweet from Jadedcat for a take on that subject and remember that adding a mod just mean supporting that mod, but supporting how that mod works with every single other mod included in the pack). Sometimes it just doesn't fit the "theme" or "flow" or "feel" of the pack. And sometimes they probably just don't feel like including it.
And that should be okay. It's why the universal configs are included, after all. Mix and match, tailor your experience. Again, I can't wait for Launcher v2 because that's where you'll really see the vision for mod plug-and-play interaction with a hopefully minimal need to tinker behind the curtain.
And because Dune is always quotable:
Arrakis teaches the attitude of the knife — chopping off what’s incomplete and saying: "Now it’s complete because it’s ended here."
--from Collected Sayings of Muad’Dib by the Princess Irulan