I remember the reason it was added to PVP was because "It is too dynamic and we want you to focus on other players rather than your combos" meaning the simpler combos are to perform the better you can expect people to perform in regards to movement and CC and other associated parts of PVP

That said, they left it in PVE because the fights are always the same, so focusing on your combos along with mechanics is the challenge, not adapting to a constantly changing environment such as PVP. Combos not being consolidated into a single button adds another layer of difficulty that isn't "spam this one button" which is preferred