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.
This allows developers to use new api calls and have their apps work on old devices, it doesn't let google break existing apis without breaking apps. They aren't like recompiling the whole play store against every new google play services build or something.
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.