The issue has several factors involved, not all in Google's control.
First off, there is the vendors... I don't know how other vendors are, but with AT&T, they take forever to roll out new versions for their phones. I assume the phone manufacturer's as well as the service provider's like AT&T and Verizon all have to customize the new versions to add in their mandatory, non-removable bloat-ware crap that they force their users to have installed on their phones. So for instance, when Google released Lollipop, then Samsung has to get a hold of it, and add in all their Samsung crap-tastic apps that can't be removed and roll it all into a custom ROM image. Then they hand off to ATT, who then adds in all their useless apps that nobody uses or wants (or don't work until you pay more $$$), and build their own custom ROM that rolls in Samsung's ROM to theirs. Then they might push it out over the air, but by then, Android has had several new minor releases in the meantime, so the version they are pushing isn't even close to the latest build version of Android anymore.
The other factor is the users themselves. And it's not always laziness on the user's part. Here is what I mean. Since the manufacturer's want to lock down the phones to the point that you can't do anything cool with them that you really want, and they only let you do what they want you to do with them, the only real way to take control of YOUR device is to ROOT it. So a lot of users root their phone, and then customize it, either installing a custom ROM image, or removing bloat, or enabling WiFi Tethering, or other feature that doesn't work out-of-the-box because it's been crippled by the service providers. But once you root the phone, it's a major PITA to update it after that. Because the service provider updates don't work on root (or will remove root if they do work), most people with rooted phones don't take the OTA updates. It doesn't help when each OTA update includes more and more modifications and locks to try to keep users from rooting their phones in the first place, not many people want to take the chance to un-root, update, and then re-root their phone, because they might not be able to root it again after the update, or will have to learn a whole new different rooting procedure than the last one they used.
I know some of you will say, well if you have root, then you can install newer ROM's yourself without waiting for OTA updates. Sometimes this is true, but other times not so much, since the service provider's and phone manufacturer's (AT&T and Samsung I'm looking at you), go to great lengths again to make sure you don't actually own the phone you are buying and so LOCK the boot-loader, and install KNOX and everything else they can think of to keep you from making things as easy as they should be when it comes to updating your phone through other channels other than them.
None of this is Google's fault, but the phone manu. and service providers. They are the ones who want vendor lock in, and customer lock-out of these devices. I sure hope that Google doesn't FORCE mandatory OS updates moving forward, since I'm always running a rooted phone, I don't want to mess with all the head-aches attached to updating and re-rooting every time they come out with a minor new build. It would be one more thing I would have to fight with on top of the other things mentioned.