Why do we have the different rules for in line, leg side, and off side for LBW.

Interesting question, Jon.

My guess (and it's only a guess) is that:

The batsman's legs are normally positioned outside of the leg stump. If LBW was allowed 'as long as it would hit the stumps' you'd get bowlers, bowling 'round the wicket' yorkers, that would have teams out for 15