There could be social value to other forms of murder. For example, if you blow someone up with fireworks, many people may be entertained. If you murder someone who wears their pants with their butt showing by shooting them in the butt, you may deter future fashion felonies. If you kill someone then feed them to whales, you are supporting wildlife…probably get some naked PETAns to come out and take a picture with you.
Seems like it would be a lot of work to decide which forms of murder (and after) deserve what kinds of sentence reduction.