There are too many market drivers to come up with just one reason. .40 S&W prospered for several reasons, but the advent of Production, Carry Optics and PCC (all 9mm dominated) in USPSA has ushered in a decline is the use of .40s. The FBI leaving .40 S&W has ushered in a decline in the use of .40s. The improvement in the factory loadings of 9mm has ushered in a decline in the use of .40s. The CCW crowd desire of small pistols has ushered in a decline in the use of .40s. The .40 was largely responsible for the decline in the use of .45s.
The 10mm (at a higher pressure and efficiency than the .45 ACP, even in +P dress) is the most powerful economical and available factory guns and ammo auto pistol cartridge in the market. When you can have 400 horse instead of 300 horse, some just like it more. It is also (in many states) legal for hunting while .45, .40 and 9mm are not.
Yes, I like the 10mm, always have. My 610, Razorback, G20SF and AR10mm are powerful, accurate and fun. 9mm is my most used caliber followed by .223, and I only have 2 .40s left with a bunch of components. But if I am not going to shoot 9mm, I just go straight to 10mm. Of the calbers I own, the .40 gets used the least since about 2015. Prior to that, it was the caliber I shot the most for 20 years or so.
The 10mm (at a higher pressure and efficiency than the .45 ACP, even in +P dress) is the most powerful economical and available factory guns and ammo auto pistol cartridge in the market. When you can have 400 horse instead of 300 horse, some just like it more. It is also (in many states) legal for hunting while .45, .40 and 9mm are not.
Yes, I like the 10mm, always have. My 610, Razorback, G20SF and AR10mm are powerful, accurate and fun. 9mm is my most used caliber followed by .223, and I only have 2 .40s left with a bunch of components. But if I am not going to shoot 9mm, I just go straight to 10mm. Of the calbers I own, the .40 gets used the least since about 2015. Prior to that, it was the caliber I shot the most for 20 years or so.