Simply because it is the standard. I am not aware of a single wallet which derives P2SH address at m/49/0/x by default, while there are hundreds which follow the BIP39 standard of m/49'/0'/x'.
Just like there is nothing stopping me deriving a single sig wallet at m/3894329'/284760'/1609266' and backing up my derivation path, it is much safer to just stick to the standard m/84'/0'/0'.
Ah ok, yes that is a fair point. However, I think so long as I backup the derivation path I should be ok. This thread will serve as an extra back up
