If they legalized it there would be a whole new tax base for the governments ongoing "profit center" search. Sure it is a weed, but few people would bother to grow it if they could just buy some instead without fear of retribution.
And ... on the flip side, think of the $$ the govt. could save in the incarceration of sellers/users it spends currently ! "Windfall Profit" comes to my mind :>
The biggest reason the fed will never legalize it is that in doing so it would violate treaties already in place with other govts.