There are 2 plausible arguments for why Glock will not release the 9mm version of the Glock 42.
First, releasing the 9mm version of a single-stack, super compact version of the Glock 42 can be thought of as a sort of sales "Trump Card." Who is to say that Glock sales will not take a plummett downward sometime in the future? Several gun manufacturers and companies are beginning to replicate the Glock-style, striker-fired internals and designs. With the increase in these Glock knock-offs, Glock may begin to see a drop in sales and revenue. They may have to cut costs in materials and production. If such a scenario happens, introducing such a pistol would help boost their sales and earnings.
The other part of that argument could possibly be that introducing such a gun is probably a great move for Glock and the earnings would be staggering. Sales would skyrocket and Glock would make a fortune for a long time to come. I personally think that Glock releasing a single-stack, compact 9mm is a great business move, but their marketing department may have other ulterior motives.
Like I said, this is just my opinion. Other companies have waited too late until releasing good products when they should have released them sooner, thus committing corporate suicide by waiting to use their "Trump Cards" at the last minute.