fastbolt
Member
I am not sure that is the reason. S&W and some other manufacturers of polymer competitors of the Glock are certainly selling them with thumb safeties. I guess Glock can do without that business.
There's really no way to know, definitively. Glock corporate is pretty guarded about many of their business decisions and reasons for doing things. It's not at all uncommon to hear their LE reps or armorer instructors claim they have no idea why something was done, or why some change was made, as the company hadn't passed along any of the info to them (sales/LE support people).
Of course, that's not "proprietary" just to Glock, as other companies sometimes do much the same. It's certainly not uncommon for everyone to communicate with everyone else inside a gun company. Not everyone needs to know everything in order for the business to run day to day.