Lee Grant said:
Because a THP playing in the technical position of LHP is less likely to stuff up than a specialist LHP playing in the strength position of THP.
Correct - and because THP is a more physically demanding position than LHP.
As a LHP your job is to steady your own feed and hammer the shit out of theirs. Locking out and holding on your feed is far easier than trying to push someone around.
When you're a THP, your job is to bunker down and hold the bastards off no matter whose feed it is. On the rare occasions that you're experience scrum dominance on the right hand side, the LHP is still probably finding it easier than you.
I don't mind saying that I find LHP a shedload easier than THP, but like Alexander I haven't spent a lot of time there.