I don't know why. Lack of space on some phones maybe, or they could be configured to autoupdate via wifi only but never see wifi. Whatever the reason, if you use google play services, you'll see a few devices with old versions that don't update even though there is a newer version for that OS version.
(∗) for the occasional banking/railway app
It says to me that they expect it to be super expensive in comparison to other stretch-goals, and that it's not a core goal of the project (and the latter should be true for literally every stretch-goal, in any kickstarter ever).