I'll say it more directly this time.
C/ER/PPCs are horribad against tanks, air, vtols, and only good against BA on the ground.
Tanks are shaped differently than mechs, their hitboxes are much more separated and wide, with more armor per section due to less areas to cover.
Add to this that you can only really damage the hull after you burn through a single side with splash damage never really taking much effect due to the shape, nor helping due to the nature of the system, and LPL are far better at taking them out. The long reload time of PPCs doesn't help either as tanks can simply turn their sides and if you're going through the turret its going to take even longer.
Constant damage to a single location is the key to scaring off and destroying tanks, CERPPCs are horrible at this.