Madecov actually hit on the most relevant factor as to why glocks caught on, when in fact they were not the best available pistol on the market. Glock used some excellent marketing strategy by approaching firearms instructors at police academies and giving them free loaner gun toys to play with and providing attractive pricing to police departments, the rest is history. Police departments probably would have done better for their officers to look at everything available on the market, which most departments will just not spend the money on, and make a decsion based upon comparison and suitability to their particular needs.
By the way, don't know about other areas, but interesting to note that this region seems flooded with used glocks that police departments have apparently gotten rid of.