Because most people prefer TiC, and there's very little reason to have both those mods in the same pack.
Uh, they're not even close to the same thing, and there are plenty of reasons to have both of those mods in the same pack. On top of that I'd take Dartrcraft over TiC every day of the week, it does much, much more.
I have to assume you've never used one of those two mods if you think there is much overlap between them.
Tell me about force pack equivalents in TiC?
Tell me about force rod equivalents in TiC?
Tell me about force grinder equivalents in TiC?
Tell me about force mitt equivalents in TiC?
Tell me how you can build mob spawners with TiC?
Tell me how you generate power with TiC?
Tell me how you grow trees with TiC?
Tell me how you fly with TiC?
Tell me about TiC item transport?
Tell me about TiC crafting?
Tell me about TiC mob handling/moving?
This list goes on for quite awhile.
Let's pretend that both mods do exactly the same thing, your argument would still be invalid because there are lots of mods that do the same thing and still coexist in FTB packs.