Turbofans and propfans are, in concept, quite similar: both have a turbojet core, plus one or more additional turbine stages that drive a fan(s) which blows air around the core (this is known as bypass air, because it bypasses the core rather than going through it). The single defining difference is that, with a turbofan, the fan(s) is enclosed in a large cowling (hence the alternative name "ducted fan", as the bypass air passes through an enclosed bypass duct), while, with a propfan, the fan(s) is left un-cowled (hence the alternative name "unducted fan").
However, in practice, there is one additional, rather odd, difference between most turbofans and most propfans. Most turbofans operate in the tractor configuration, with the fan(s) placed out in front of the compressors and pulling the engine (and the airplane attached to said engine) through the air, while most propfans operate in the pusher configuration, with the fan(s) aft of the turbines and (surprise, surprise) pushing the engine (and, hopefully, aircraft) through the air. There are exceptions on both sides; the General Electric CJ805-23 turbofan, powering the Convair 990, operated in pusher configuration, while the Progress D-27 propfan, which powers the Antonov An-70, operates in tractor configuration; however, both are exceptions, and the tractor-turbofans\pusher-propfans rule holds true for the vast majority of engines in both categories.
Why is this?