Part of the story is that over the years since c. 1899 (the first S&W M&P) or 1909 (the advent of the "new" Colt action), the S&W lockwork has undergone a continuing series of product improvements, not all of which are reflected in the "change" numbers. Very few of these were even noticeable on the outside and S&W liked it that way. Anyone with a Model 1899 can take a look at such parts as the trigger rebound and the cylinder stop, to see what I mean.
Colt, once they had a reasonably successful system, "froze" it and ceased even trying to improve the internal design, though they made cosmetic improvements for marketing purposes (e.g., ramp sights, ejector rod shrouds). Then when Colt did decide to change, under economic pressure, they no longer had the capital to ride out the inevitable resistance to change and work out the "bugs."