Because there’s no Guards Operating Panel at every door within the carriage, and the problems that will create on very busy trains. The guard will have difficulty reaching a GOP within the carriage.
I know, how on earth do Northern staff been doing it for 6 years on the 195s and 331s!
SWR guards wouldn't even have to move from one side to the other that much, for example on the Reading line: ("outside" referrs to not island platforms, "inside" refers to island platforms)
Reading - Either side
Earley - Outside
Winnersh Triangle - Outside
Winnerish - Outside
Wokingham - Outside
Brackell - Outside
Martins Heron - Outside
Ascot - Inside on P2 (P1 both sides?)
Sunningdale - Outside
Longcross - Outside
Virginia Water - Outside
Egham - Outside
Staines - Outside
Feltham - Outside
Twickenham - Inside (mostly)
Richmond - Outside
Clapham Junction - Either
Waterloo - Either
(I used the stops that the 2C68 2009 Reading to London Waterloo stops at, and considered both directions)
A guard having to cross the train 7 times at most over an hour and a half feels like quite little, the smallest time gap is 3 minutes, which is surely more than enough to move 15 metres (that's 0.08 metres per second).
Surely that's possible for someone to do, even on a busy train.