That's just not the way the trees are designed. Madness is designed around Force Lightning both as a filler spell and as the trigger for Wrath procs, which make Crushing Darkness hit hard and tie the spec together. Lightning is designed around Lightning Strike as a filler that gives you the procs you need.
If they wanted to give the lightning spec spammable force lightning, they would essentially need to re-design the entire tree so that procs came from force lightning instead of lightning strike. And that would make lightning strike completely useless for any spec.
Good idea- let LS die, and make wrath just proc crushing as well as finish crushing's cooldown. Make crushing stack, or, make it so if you use crushing on someone who already has the dot- it instead does all its damage at once.
Suddenly- burst damage.