The offset dish (in most cases) is easier and cheaper to manufacture than a prime focus unit
When it comes to the process, the best prime focus antennas are spun on a large revolving former and pressure is applied until it takes the same shape. This is expensive in both time and labour
Offset dishes are usually pressed out in the same way as sink units, waste bins and other household objects, by the million, so a domestic dish can be made for peanuts. The slightly lower gain can be put down to a number of factors, poor manufacture, poor packaging, Parcelfarce, bad installation, etc etc, and usually the final user may never notice unless he sees a picture from a decent one.
You will see many posts with people wondering why their signal drops out when it rains, or when the wind is blowing, the forums are full of them and it is not always apparent from the post as to what is happening, but I bet that nine times out of ten, something has not been put up right outside, rather than a fault of the manufacturer.