Honestly it was mostly just changing tastes. Also, new guns are always coming out...
My first gun was a Kahr P9. I didn't like the grip texture or the felt recoil. Then I had a Kel-Tec P32 (Don't laugh, I was young and dumb). That was unreliable and underpowered. Then I had a Ruger SP101 2.25". That was fine, but I didn't have a good holster for it, and the cylinder bulged against the holster and chafed my hip. Then I had a S&W 642. That was fine, but eventually I gave it up for an M&P .40 S&W full-size, which was too far in the direction of size and capacity and hard to conceal well. So I got an M&P9c, which was great for a long time.
Then I added a Ruger LCR for when the M&P9c was too heavy (mostly when running). Then I added an M&P Shield in 9mm, which has replaced the M&P9c because it's thinner, lighter, and just as comfortable to shoot.