Why were GIs so popular?

Answer 1

From the perspective of a former GI:

Soldiers engage in pretty common behavior. Being cooped up for several months during training, and under stress from the time they join until they DeROS or retire, any opportunity they have for down-time, they use to blow off steam. They do this by spending money and partying. This attracts the attention of townies. Everyone likes to party, and when it is the GI who is throwing money around, there are those who will attach themselves, going along for the thrill of the ride.

This other sounds a bit callus, but is no less true:

Another situation exists as well. Emigration to the US is not always an easy thing to accomplish. One way to ensure that it happens and does so without problems is to marry a citizen. Currently, the US has citizens in over 60 countries around the world in the guise of service members. It is not uncommon for natives of these countries to attach themselves to young, naive soldiers, convince the young men to marry, and bring their bride to the states. Instant Immigration.

From an objective point of view, most US service members are young, healthy, and fun to be around. This combination makes for a high degree of popularity.