It has to do with the real-world geometry of the parts. If the engine is on the front of the boom (BV-141 style), the prop is adjacent to the forward part of the pod. You need a good 8" to a foot clearance between the prop tip and the pod side for reasons of cockpit noise and vibration. The prop arc tends to fall somewhere between the pilot's ankles and knees in most reasonable arrangements. That section is about 80% of the width of the widest portion of the pod, which forces the centerlines of the pod and boom farther apart. The wider the pod, the worse it gets, with a side-by-side seating pod being the worst case. The farther apart the centerlines are, compared to the span, the more aileron power you need, and the longer the tail boom has to be, to preserve lateral control power.
If the engine is, instead, a pusher installation behind the occupants, the width of the boom is the limiting factor on the spacing of the centerlines of the pod and boom. Since the boom can be relatively narrow in plan view, the centerlines can be relatively close together compared to the BV-141-style case. Everything gets smaller as a result. It's just lighter and easier.
Do some scale drawings of both configurations, including pilot and passenger, and you'll see what I mean.
The BV-141 configuration also has another issue when applied to small airplanes: As I said earlier, the prop arc tends to fall between the pilot's ankles and knees, and that presents a danger in the case of a thrown blade. You either need to reinforce the pod against a blade strike, or accept the danger and odds that the blade will actually hit the pod.