For starters, most of the early .40 pistols were hastily converted from 9x19mm designs and that's understandable, since part of the design brief of the cartridge was that it fit in 9mm blasters. The USP was designed around the cartridge, and the Smith & Wesson Third Gen autos were beefy chonks that were easily up to the job.
SIG Sauer wisely realized that the original P-22x architecture, with a separate breechblock held in a stamped slide by a roll pin, would not be up to the job and came up with the milled slide P-229 for the role.
On the other hand you had pistols that were really marginal for the chambering. Beretta 96's ate locking blocks like popcorn, relative to their 9mm siblings. Even the ubiquitous Glock 22/23 pistols had issues, being more prone to feeding issues from improper grip or a mounted weaponlight than their sewing machine reliable 9x19 progenitors.
Also, in typical duty loads there just wasn't enough difference in terminal results to justify the loss in ammo capacity and the reduction in shootability. No matter how good you shoot .40S&W, you shoot 9x19mm better.
That being said, it's not like the chambering doesn't work. It works great. It was specifically designed to meet the current benchmarks in terminal performance; it was "taught to the test", to steal a term from education jargon. You can pick up some really great LE trade-in SIG and HK pistols in .40 for a song these days. Ammo's more expensive, sure, but if you aren't going to be shooting it a ton, who cares?
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