Why is there really only one basic design for passenger airplanes?
For most transport aircraft, aerodynamic efficiency is the key parameter as it allows lower fuel consumption. The layouts you see most often are the most efficient known:


For jets low swept wings with engines mounted under and ahead of the wings and conventional tail is most efficient known layout. Tail-mounted engines used to be common on smaller jets, but as that puts lot of bulk far aft, they are worse with respect to Whitcomb area rule, worse for weight and balance, and wing-mounted engines also help damp flutter. So new regiojets are switching to engines under wing too.
Propeller aircraft fly slower, so they have straight wings. Low mounted wings are slightly more efficient, but then the plane has to have long gear to maintain sufficient ground clearance for the propellers, so high wings are more common. T-tail is then simply used to put the elevator above the most turbulent wake of wing and engines.


The only deviations from these three basic layouts are on special-purpose plane that have other more important concern. The most notable is military cargo planes are generally high-wing so they can sit low on the ground for easy loading and unloading using built-in ramp. Their marked anhedral is to avoid overstabilizing the aircraft in roll as both high wing and wing sweep increase roll stability. Earlier regiojet designs used tail-mounted engines for the same reason; to sit lower on the ground so they can be easily loaded via built-in airstairs and these are not too heavy and bulky.