Umm I would think the obvious reason is that Republicans don't want to go up against people who would vote for marijuana legalization in an election. Make it federally legal and keep the liberal stoners on the couch on election day...

Me personally, I'm as liberal as you can get and I won't ever vote for a marijuana legalization bill that is a half-measure, that is to say, one that doesn't prohibit employers from testing their employees for the proposed legal substance. If it's going to be federally legal, it should be legal for everyone, not just people who have chill bosses.