That's why...taught by S&W. THe M&P is a good gun, but doesn't have near the track record of a Glock. Give the M&P 10 more years and a few deployments and then compare.
(I know, fellow 1911 enthusiasts will start to chime in here and a 1911's reputation cannot be discounted. However, the 1911 is a finicky girl and does need cleaning & maintenance much sooner than a Glock. Not to mention the greater worldwide availability of 9mm and the capacity/interchangability of Glock mags.)