One of the main reasons is older shooters are comforted to carry handguns that have saved their lives in the past. It is hard to quit winners. And one should not discount those years of contact with a certain handgun design, in which the shooters have learned the gun's operation intimately. You often hear the comment, "I could field strip it in the dark." In times of great stress it is comforting to be armed with a firearm that you know everything about.
And there are new shooters who come along and decide that if a certain gun design has worked for people over all these years, it must be good. I have always liked the fact that the 1911 .45 ACP was battle-tested successfully long before I ever laid hands on one. Conversely, I have always thought it was a really bad idea to have to be the first one to test a new gun design in an actual gunfight. Too many bad things can happen.
Today we are blessed with a wonderful assortment of handguns that may be suitable for personal defense. And none of my remarks lauding the praises of the older designs should be taken to mean that I am against new gun design. I wouldn't have the job I have if that were the case.
The smart thing is to experiment with a broad assortment of defensive handguns until you find the one that is best suited your needs. Old or new, it really doesn't matter as long as you find what really works for you. Once you do find that perfect defensive handgun, buy it, no matter what the cost. In fact, buy two or three so you'll have some spares. The important thing to remember is that personal defense guns shouldn't be some sort of fashion fad, where you run out and get whatever is new. Do whatever it takes to get what works for you and then stick with it.
One of these days, if you're lucky to live long enough, you'll be saying, "Why should I give up this old pistol for something new? It has saved my life and we have a history." In other words, if it ain't broke, don't fix it!