Ok, you have to realize that "the police" is a large and poorly defined idea. There are thousands of police departments and agencies in the U.S. and they may not have identical reasons for how they pick their weapons.
That said, in very broad strokes, the thinking in the early 1980's was that cops were "outgunned" with six shot revolvers. Whether this is true or not can be debated.
The U.S. military had just switched to the Berretta 92 at the time so the idea of the "wondernine," the high capacity 9mm, was big.
Modern JHP's were also becoming more common at the same time and the newer generations eliminated the feeding problems that were common with early versions and older designs of auto pistols.
The big switch to semi-autos in many PD's was in the 80's. The early contenders were the Berreta 92, the S&W 59 and later guns, and the Sig 226. Later the Glock was intoduced and pretty much dominated the market. The switch to .40 happened about a decade or so later.
As to "why not the 1911" remember the 1911 wasn't always held in the same respect it is today. It was largely looked at as heavy, inaccurate, and something that "kicked like a mule." The single action design with a visibly cocked hammer was also unsettling to many people, including many police administrators.
Besides, when the trend was for "more firepower" the thought was that 15 or so 9mm rounds was better then 7 or 8 .45 rounds. That would have been only 2 more rounds that the six shot revolver, after all.
