1. Law of Christ is too good to be true. It can't be that easy. Just trust God and love Him? The Law of Moses is just hard enough... if I just cut it down to 10.
2. Law of Christ does not have solid boundaries. The Law of Moses is easier; it has a list of easy to understand dos and don'ts.
3. We do not trust the work of the spirit in our lives to live out the Law of Christ with love and so fulfill all righteousness (Rom 8:3), so we revert back to what we think we know works: Law of Moses.