Why does my insurance company get credit for my CPP Disability wen they had denied my claim, and that was the only reasonI applie for CPPD?

Your insurance company's right to deduct CPPD comes from the terms of your policy. In other words, there are terms in your policy that allow them to deduct it.

If they've already cut you off of LTD, then they should not be entitled to any of your ongoing CPPD payments, but they may still be entitled to part of your retroactive lump-sum cheque from CPPD, if that CPPD money is paid for a period during which you were also collecting LTD (creating a "double-recovery" period).