Why didn't S&W build a Model 52 in .45 ACP?
Two reasons. First, the M-52 isn't big enough for the .45 ACP.
Second, tooling up for another frame size was extremely expensive at the time and S&W had their hands full meeting demand for their existing product lines. Whether the market for such a gun would have been there is debatable, as Colt and military surplus 1911's were plentiful and inexpensive and even at the time, Bullseye target shooting, the target market for such a pistol, was in decline.
S&W's first .45 Auto pistol was an experimental gun made by Tom Campbell, one of their engineers. Campbell was a successful IPSC shooter and S&W sponsored him. Scoring in IPSC matches was caliber-sensitive, hits outside the main scoring area counted higher with 'major' calibers like .45 than they did with 'minor' calibers like the 9mm. Campbell had been competing with S&W 9mm pistols, but Campbell both wanted the higher scoring ability of a .45 ACP pistol and also wanted to remain loyal to his employer and use a S&W product. His solution was ingenious.
Campbell took 2 Model 52's and cut them into 2 pieces each, along the verticle axis. One gun was cut with the left half thicker and one was cut with the right half thicker. He then welded the two thick halves together and hand-made the parts to make the hybrid in .45 ACP, such as barrel and magazines. The gun became known as "Supergun" and was quite a sensation.
S&W didn't really see an economic marketing opportunity for a commercial .45 ACP pistol until the 1980's. They were upgrading their first generation series of service-grade semiautomatics (the Models 39 and 59, basically) and could see the demand for a .45 ACP pistol for police and civilian use in their traditional double action-single action system. Computer aided design and manufacturing was developed then, and that's where we got the first commercial production S&W .45 pistol, the Model 645. The 645 and then third generation .45 and 10mm pistols resulted. Demand for a single action version just wasn't there, and the single action variants, using this basic frame, like the 745, 845 and 945, have been produced in only limited quantities. The 745's, the first single action variant, was a really slow seller. It seems that most folks wanting a single action .45 ACP pistol prefer the 1911 system. Even S&W makes 1911's now.
Probably more than you wanted to know, but there it is.