[carguychris]

Two other reasons that haven't been mentioned...

Most Magnums originated as revolver rounds and have rims. It's harder to make rimmed cartridges feed well in an autoloader, and they stack in an arched shape, which makes reliable high-capacity magazines difficult to design. One of the major reasons people prefer automatics is their ability to hold 10-17 rounds vs. 5-7 rounds; if a pistol can only hold 7-9 rounds, the advantage over a revolver is much less significant.

Autoloaders generally can't utilize lower-powered alternate cartridges for low-recoil practice unless the shooter is willing to cycle the action manually and/or continually clear jams. Sure, you can use lower-powered rounds in some pistols by swapping recoil springs, but most shooters don't have the initiative or desire to swap parts before they can switch ammo, and one has to figure out which springs work with which loads before this is a viable strategy.