My "guess" (strictly IMHO) is that Glock is fully aware and capable of putting rails on the 26/27 frame... However, the cycle timing metrics for a 26/27 is much tighter than for the larger frames. I have known first hand firearm factories that "elect" not to build certain configurations simply because smaller configurations are more dependent upon the shooter grip/ability for reliable cycling.
Most likely, 99.9% of those on this forum would grip securely enough to enable reliable cycling with the myriad of possible light/laser/bayonet configurations that could end up on a 26/27 rail and result in an inordinate amount of customer services "issues" that would be costly when it really isn't a "problem" with the handgun.
On the other hand, it might add "points" to the holy BATFE point grail allowing imports and it could be as simple as adding a rail would limit import ability.
"No good deed goes unpunished"