Glock is a for profit business. As with any business the drive to develope new products is dictated by the sales, profit margin, and R&D costs. Currently Glock has the LE market on handguns wrapped up. Close to %60 of US police dept use a glock. Plus a strong foothold in the civilian market. Not to mention international sales. Look at all the glocks in the sandbox right now.
The small caliber rifle market is crowded right now in the U.S. There are several AR manufacturs right now. Sig is coming out with a new rifle. There are a lot of moderately priced AKs. Plus there is still Ruger and Kel-tec. Very crowded market, low profit margin.
R&D is expensive your goal as a business is to maximize profits. Why spend the money to enter a very crowded market for small caliber semi auto rifle?
As with any business, until Glock starts losing a significant market share development of new products will be minimal.