This discussion is pointless. Why not ask why the UK drives on the wrong side?
Would yield the same 20 pages of flaming.
I think that's totally different.
With imperial vs metric, it's easy to choose and both can easily be used, so long as the units are defined. As far as which side of the road to drive on is concerned: everyone on the same road network must do the same, otherwise chaos would ensue.
Why hasn't the UK changed to driving on the right hand side? Probably because it would be too costly and the sudden increase in accidents that would follow when people forget.
The UK changed to metric, as far as trade and industry are concerned, yet all the road signs are still in imperial, most likely for the same reason as above. I don't see why the US can't do the same, even if it is slowly phased in over a long period of time.