Glocks aren't perfect and I'm not a fanboy for the brand. When I was looking for a carry pistol, I looked at a lot of things and ended up with a Glock...
Here's why I found the Glock to be the better choice vs the competition for me...
1. Reliability - Proven in the field for years. You can say the same for the Beretta and SIG in terms of track record over many years.
2. Accuracy - Proven again. You can say the same for the FNS, XD, Beretta, M&P, and Sig.
3. Capacity - power/weight ratio - Glock is great here, same can be said for the Beretta and SIG in terms of ability to go to higher capacity, i.e. 32 Round mags if needed. Not sure about FNS, XD, M&P. Glock appeared to be more streamlined and easier to carry vs the competition.
4. Simplicity of Design - I think most people overlook this. Glock is the simplest of all the competition mentioned. Fewer parts = less to fail. You can completely detail strip a Glock with a single punch. Sig is comparable, but the other designs, not so much.
5. Availability/Cost of Parts - Glock wins here. Very few parts and very cheap to acquire.
6. Shootability - The Glock Trigger can be easily modified to get what you want. Not perfect, but pretty damn good. Trigger reset is very short. Glock is not the best in terms of ergonomics, but can be improved with an adapter. Given the other advantages, I can live with ergonomics that are not as good.
7. Designed for Reactive Shooting - nothing to think about with the Glock, just draw, point and fire. Beats the external safety designs in this regard and has a better trigger than the hammer fired DAO designs. No need to think about decocking in a high stress scenario.
These were the criteria I used to make my choice. YMMV. I think many of us who have chosen the Glock for serious business (not range fun) have gone through a similar thought process, which may explain some of why they are so popular. Adoption by so many police agencies is another.