Here is my understanding....

Wireless ones need to have the sensor close to (and pointing at) the computer, and having the sensor on the back wheel would require you to put the computer behind the seat or something like that. But it would "work" though.

For wired ones, they just simply don't send you a wire long enough for the back wheel because most want it on the front.

I have a cheap Sigma computer, and I wanted to put it on the back wheel. My bike shop was able to order me a replacement sensor with a longer wire for $9. I think it was called a "rear wheel sensor kit." It works fine. I had to use a bunch of scotch tape to get the wire to run from the back wheel all the way to the handlebars. Mounting on the front wheel is MUCH easier.

That's the way I see it anyway. I could, of course, be totally wrong about everything I just said though.