You get the most benefit from traction up front for cornering and braking, which everyone does, compared to climbing which is done less frequently especially in flat areas. The other thing to consider is that since the rear tire carries more weight, it gets more benefit from a tire with lower rolling resistance. The weight on the rear tire and acceleration applied to it wears it out much faster than your front tire, as well.

These two things are a big factor on some new bikes, especially Specialized since they have so many of their own brand of tires to pick from. It's normal to see a 2.2" front and 2.0" rear, with more aggressive tread up front.

I'm with Thor. I have more tire clearance up front than rear, so I'm limited to an honest-to-size 2.25" or slightly smallish 2.3" in the rear. I currently have Specialized Ground Control 2.3" front and 2.1" rear, but I can fit the 2.3 in the back because it's actually more like 2.2"... so I'll do that when I wear out my rear tire.