Chicken6,
I feel I must take you to task on some of your comments. "A high wing aeroplane is more stable." Stable than what? I assume you mean more stable than a low wing aeroplane. You then go on to give an inaccurate description of how a high wing aeroplane gets its lateral stability and then make the statement that a low wing aeroplane cannot do the same. Have you never heard of dihedral? Sweepback? The effect of a T tail on lateral stability?
Your theory that the lateral centre of lift moves "downhill" is wrong. The reason that a high wing provides lateral stability is due to the fact that, when the aircraft is sideslipping towards the lower wing, some of the airflow moves so as to go across the top of the fuselage. This upwards movement increases the angle of attack on the lower wing compared to the other one, thereby increasing the lift of the lower wing. The rolling moment so produced provides the lateral stability. The pendulum effect is not really aerodynamic, it is as its name suggests, like moving a pendulum from the vertical - gravity will make it try to return to its previous position.
As to why some aircraft have high wings and some have low wings, I think the only people who can answer that question are the manufacturers. Having flown and instructed on both types, the only advantage I can think of for a high wing aircraft (apart from keeping dry in the rain if that is what is important to you), is the downwards view. This is more than negated by the appalling and potentially dangerous lack of visibility into the inside of a turn. And yes, it may be nice to stay dry when it rains, but unless you are at least seven or more feet tall, you will have to climb up to check the fuel contents and the conditon of the upper surfaces of the wing. Another major disadvantage I have found with a high wing aeroplane is the lack of pre-stall buffet due to the turbulent airflow from the wings at high angles of attack passing clear over the elevators. I am not criticising that stalling characteristics of the aircraft, merely the almost total lack of aerodynamic warning of an impending stall.
There are advantages to high wings for a transport aircraft, and there are numerous examples flying today - C5, C17, C130, C141, An 124 etc. the advantage being ease of loading/unloading, but I suspect these aircraft were not in the mind of the person who originated this forum.