I thought it was because it would be tough to fly straight and level in an aircraft designed for maneuvering, and as such, highly responsive to control input. Please correct me if I am wrong.
Seems I recollect reading somewhere that early biplanes which were good at dogfighting were also not pilot-friendly because of inherent narrow windows of attitude stability which made them more maneuverable.
It's actually not so much the ability to manuever, that is pull g's, but rather the ability to do so quickly. Most aircraft are dynamically stable. If you let go of the controls, they will settle into a steady attitude, perhaps climbing or diving, but not chaning attidue and direction of flight. Aircraft like the F-16 and F-22 are dynamicaly unstable, or marginally stable. It's takes an active control, a feedback loop, to keep them at a constant attitude. Unlike the stable aircraft, absent their computerized flight control system, they would diverge after any little disturbance. Further more, they would do so to fast for pilots to correct them, unlike those biplanes which were probably marginally stable, but with the "time constant" of the divergence/instability long enough that a pilot could easily provide the necessary feedback.
The first aircraft, the Wright Flier, was unstable, due to having it's elevator in front of the wings, rather than behind. You can actually see that in some old motion pictures of them. You can of see it in this video of a reproduction Wright B model. It's not as obvious as with the "A" model, but you can see it in the flapping elevator, especially right after takeoff.