ghostZ,
Easy answer, name recognition. While the H&K, and for sure the Sig, are quality arms, everyone is familar with the name "Glock", and to a lesser degree, "Springfield". Of course it doesn't hurt Glock sales any that they are seen in about 85% of police holsters! Not to mention the number of things that rhyne with "Glock" in rap songs.
Stay Safe,
Savage