I believe it's because they have separate components and probably refrigerant. It might be that your front AC needs to be recharged, but the rear is fine. It could also be that a component critical to the front and not the rear has failed.
The previous answer is possible, but the most likely cause is the front orifice tube is clogged and needs replacing. Both front and rear AC units use the same refrigerant, but uses different orifice tubes. Therefore the most likely cause is the screen on the front orifice tube is clogged. I have the same problem. My friend repairs AC on his job and he is assisting me now. I do agree with others to get a professional to replace the tube.