There are several reasons, mostly historical. Some had to do with the fact that 10mm guns were physically large and therefore hard to conceal or for people with small hands to use. "full power" 10mm was also "too harsh" for many shooters, being designed as it was for maximum barrier penetration.
The "light recoil" version of the 10mm round did catch on, it was simply put in shorter brass, we know it today as the .40 S&W.
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