Most of what I’ll say has already been covered, but I’ll give my reasons:
Reliability (I once ran 2000 rounds through my G19 during a 4 day pistol training class without any cleaning and had zero malfunctions).
Simplicity (fewer parts than almost any other auto pistol out there).
Phenomenal aftermarket support (if you want it, they probably make it for the Glock).
Cheap and easy to rebuild yourself.
Accurate enough for a combat handgun (and you can buy better barrels etc to accurize it if desired).
Size to capacity ratio (always a decent amount of rounds for the size of gun).
Over the years, I’ve owned and shot probably every design of handgun out there. In my experience I can say for sure that there are many other guns out there that feel better in my hand, and definitely some that are more comfortable to shoot than a Glock…however, I love the ultra simple design, and total reliability (in my experience) of the Glock.
I hate getting a gun that I really like only to find that mags are unavailable or ultra expensive…night sights are non-existent or super expensive…or, when I need a part that has worn, I have to call customer service and pay overnight air shipping ($40-$50) back to the manufacturer so they can replace a $5 part that they won’t sell to the public.
I’ve loved my Hi Powers, my 1911s and many others, but I train with and carry my Glock.
I only own one other carry gun that is not a Glock, and that one is an XD Tactical with the stupid ass grip safety bypassed.