In other words, the "norm" was that the DM was responsible for the setting, story hooks, etc. Even if using a published setting and published adventures, the idea was that the DM was running a campaign, that somethow tied them all together.
The APs seem to be designed with this approach somewhat in mind. Some more than others. But there really hasn't been much in the rules to support this. Yes, the DMG has a lot of things to help create a campaign world, but the publication of APs implies that it's not really needed. Instead, they've seemed to substitute "sandbox" for campaign. The idea that having a lot of space for people to explore and no explicit single linear path gives this illusion.
And really, that's the crux of the problem. Without an actual campaign to support the adventures, when the PCs go off track, there aren't other options for them. Most of the APs give the illusion of a campaign by existing in the same setting. And the other supplements have continued to be centered on that setting. But the APs stand alone. They function more like a video game, where there are defined limits to where you can go, and there are other interesting things, and occasionally beneficial things, for side adventures, but there is one plot.
In a campaign, there is no "one plot." If the PCs decide not to follow this plot, they find another one. You can make them as sandboxy as you'd like, ultimately there is still the one plot to return to, or you select a different AP.