Let me point you to the book Glock: The Rise of America's Gun by Paul Barrett. In the book, he argues that Glock used an effective marketing campaign targeted at police departments. If you're interested in learning more, NPR's Fresh Air had a great interview with Barrett in January last year.

First a bit of background. Prior to the "Wonder Nine Revolution" of 70's and 80's, cops generally carried revolvers.

By many accounts the first "Wonder Nine" (A semi automatic, double action handgun with a staggered "high capacity" box magazine holding more than 10 rounds) was the Smith and Wesson Model 59. This gave the promise of a handgun with more than twice the rounds of a conventional revolver, familiar double action controls, and a cartridge, which on paper was pretty similar to the .38 special, which despite inroads from .357 magnum, and to a lesser extent, .41 and .44 magnum, was still popular.

I believe though, while Smith and Wesson started the ball rolling, it was marketing genius Gaston Glock, and his Glock Model 17 which really got the Wondernine trend booming. As /u/The_Alaskan has already offered, Glock used a highly effective marketing campaign targeting law enforcement agencies. This in turn was coupled with effort to refine the 9mm cartridge, and create more powerful and effective loadings.

Prior to the 70's, most common autoloading pistols in the United States were either small caliber guns, various models of the venerable 1911a1, or european 9mm semi autos. The European 9's lacked market penetration, and the 1911 had not been as refined as it is now. Many were either well worn war surplus, or tight and sometimes finicky commercial guns designed for sport shooting. The 1911 as mass market combat handgun had not yet arrived in the form we know it now.

That left the revolver as the dominant sidearm of choice for citizens and law enforcement alike. Well established manstoppers like the 125 grain "FBI Load" for the .357 ensured a functioning and effective combat revolver, and some agencies did not give up their revolvers until the early 90's (Most notably New York City).

The semi automatic transition was born of new pistols, and later higher quality ammunition reaching the market, which in turn pushed the revolver aside as the gun of choice of both citizens and police. Without the combination of clever marketing by the gun manufacturers, improved 9mm ammunition (standard velocity 9mm ball ammo is considered underpowered, modern 9mm ammo is generally a high velocity hollowpoint round capable of deeper penetration and more damage than a standard 9mm ball round), and a need to keep up with changing technology, revolvers would have lingered in service much longer than they did.

In addition to the marketing mentioned by /u/The_Alaskan, there were also high-profile shootouts between police officers and criminals in which the difficulty of reloading a revolver vs. reloading a semiautomatic was found to be one of the factors in officer casualties. The Newhall Massacre of 1970, Pine Ridge in 1975, and the 1986 Miami shootout being the first three that come to mind, but I'm sure I'm forgetting a few in there. In some cases, the departments failed to issue speed loaders to their officers as well, further slowing the officers down.

Having lived though this reading news papers, I seem to recall a huge outcry over perceived disparity between how police were armed and the guns being used by gangs, primarily drug gangs and cartels.

But that is a recollection which could be very faulty.

Can anyone verify, correct or eliminate that?