Because it is in the rules.... LAW 36 (LEG BEFORE WICKET)
The striker is out LBW in the circumstances set out below.
and (b) the ball, if it is not intercepted full pitch, pitches in line between wicket and wicket or on the off side of the striker’s wicket,
It is irrelevant as to whether the bowler delivers over or around the wicket. It has to do with whether the batsmen has the ball delivered on their off side or on (leg) side. If it pitches outside of leg it does not matter if the ball will hit the wickets or not, it will be given not out.
However if the ball pitches on the off side and is in line with the wickets it should be given out
As to why there is the difference, it may be hard to determine, but if it was allowed to be given out with a pitch outside leg then I would say most deliveries would be going there since it is harder to hit (and defend) a ball coming from that direction.