Unlike a fixed forward firing gun there is no unison of movement with a flexible gun particularly a rear mounted one. On a fixed forward firing gun, the pilot, aircraft and gun can follow and hone in on the target as one. A gunner/observer has no control over his aircraft and has to continuously and swiftly manhandle his gun into position whilst more often than not being shot at himself. Silencing the gunner/observer was usually the first priority with an attack on a two seater.
Most of the two seaters were also slow in comparison to single seaters and had little maneuverability to aid in evading an attack. One fast diving attack was often all that was needed to silence the rear gunner. The Immelmann turn further increased success in this form of diving attack.
Although pointing your machine and thus your gun at a target was clearly the best option, the problem was always what was going on behind you. One solution was thought to be an aircraft that could fight and defend itself at the same time, a fighter with both front and rear mounted guns. In this the Bristol F2b fighter came into its own albeit with a disastrous beginning. The importance of the pilot and gunner relationship had chance to develop with this aircraft. It could compete on equal terms with even the best single seat fighters with that added security of a having what was once a very rare thing, a seasoned gunner in the rear!
Although as Bristol Scout says any well handled machine with a cool gunner had to be treated with respect most two seat machines were considered easier meat than a single seat fighter, (check
Manfred Von Richthofen score sheet, you'l find a lot of two seaters there

)
Also of interest is the DH2 which Bristol Scout mentions, the first ones did have a flexible gun mount. Pilots didn't like it and quickly found ways of fixing it in place.