They'd probably need a slimmer slide, in addition to the smaller frame, to compete with the narrow Kahrs, KelTecs, Rohrbaugh, etc. This means they lose the production ease they have with, for example, the 26, which is a very direct descendant of the 17.
Also consider the Glock 36, the "slimline" .45. It is the slowest-selling Glock with the exception of the GAPs. Glock may look at that near-failure and consider a slimline 9mm just as risky.