It's difficult to answer 'why', but you can certainly add it yourself (and I'd recommend just that, if you plan on doing much with IC2) http://immibis.com/mcmoddl/
My guess is Immiblis is not supporting it very rigorously.
FTB only "tries" to use mods that are persistently supported by it's author.
According to that thread Bagman linked Xboney offered to "fork" it but wasn't taken up.
