Haha, it was done to enable  signature aggregation aka Schnorr  signatures to combine multiple signatures into one single proof.

https://medium.com/badger-blockchain/schnorr-signatures-following-in-the-steps-of-segwit-a4c515c02e22

Had seen long threads talking about the implications in removing the sign bit but had to record some of them. 

 Schnorr sig aggregation would have worked fine with the old serialization.