Basically 5 mods do not support RF natively. IC2, Buildcraft, Forestry, Factorization and Railcraft. Continuing to place mods in the main packs that do not support RF makes no sense. We end up going "everything can be run off this one power system, except these 1 or 2 mods which need their own thing." MJ can fit because the conduits convert to MJ. But IC2 and FZ just don't fit. When IC2 and BC were the mods that everyone used for power the packs were built around them. Now mod devs have mostly moved to RF. Continuing to include things because "tradition" is silly politics and leads to stagnation. It leads to perfectly good mods getting overlooked as people cling to the "traditional" By splitting the energy systems up into seperate packs we allow for more variation. Just think: a steampunk factory pack with Steve's Carts, Steve's Factory Manager, BC and FZ, a High-tech pack with IC2, and GT,a magic pack with TT, Forbidden Magic, and many of the Thaumcraft addons, an RPG pack with Witchery and Botania, new mappacks based around different mods with tutorials and quests. (No this is not a list of the packs we are definitely creating) Once we stop trying to shove all the mods into one pack and dealing with the tech support nightmare we have more room to branch out. And we leave room for the community to make packs that aren't just the same as our packs with 1 or 2 differences.
IC2 and BC aren't going to die just because we move them out of the primary packs and into separate packs.