Why are rear wheels dirtier?
As mentioned in the other answers, there could be several reasons that your rear tire is always muddier than the front after a wet commute.


The front tire splashes mud on the rear of the bike.
There is more weight on the rear tire, causing it to sink deeper into mud.
Your power is being transferred to the rear tire, creating more friction than on the front.
You're probably using your rear brakes, causing your rear tire to slide.  If so, it's probably better to use more of the front brake.


One more thing... the mudguards are probably not keeping your wheels and tires clean.  They're there to keep you from getting splashed.