Because most of the "mods" that use MJ are, or began their life as, addons to Buildcraft. And there are really only five of them - Railcraft, Forestry and ExtraBees, Thermal Expansion, and Buildcraft itself. Five mods out of the 40+ mods in the FTB pack, that's not really all that much, is it?
On the other hand, the following mods deal with EU: Industrialcraft, Nuclear Control, Advanced Machines, Advanced Solars, GraviSuite, PetroGen, MFFS and GregTech. That's eight mods, three more than the Buildcraft side.
And even counting those two groups together, that's still less than half of the mods in the modpacks.
Your complaint is extra ironic, because back before the big update to 1.3, MJ using infrastructure basically did not exist. Everyone did everything with EU. In fact, people developed extra mods for the sole purpose of being able to run their quarries with EU, which was the one single machine that actually required more than a Redstone engine and was desired by most players. Oil refineries were shunned because 9 out of 10 people were too incompetent to set up proper cooling for combustion engines (it was harder back then, without liquiducts and aqueous accumulators), and well, since everything ran with EU, why bother?