Because having two is redundant. Would you make employees at a store wear one nametag on their chest and another on their back?
Yes, if they travelled at upwards of 30mph. Number plates and name tags are a really baffling comparison, essentially requiring you to think of your interactions between the two as somehow similar.
You're needlessly making people drill holes in their front bumper and messing with the appearance the car's designer intended it to have.
I can't picture a car without the front number plate, and I imagine I'm far from alone in that. Although with aesthetics I can now at least see some reasoning/logic behind the opposition.
If you're viewing a moving vehicle from the front, it's likely moving too fast for you to take down its license number anyway; whereas if you're sitting behind a car in traffic or following it, you can easily get its plate number.
Sorry, but you can't escape the fact that a front number plate will allow the viewer a better chance of seeing the number plate, if only to see a few letters before they go past and quickly turning around to catch the rest.