Go back to the 70's and 80', witness the insane popularity of songs by the Jacksons, and in particular how popular Michael's songs were.

He also did duets with other artists and I believe he had hand in songs that became popular with other artists.

That's all, really. The Jackson's had their finger on the pulse of pop. And I say this as someone who never listened to their music intentionally, even I was very aware of it (and will admit much of it had a strong appeal).

Edit: Pop is tricky to stay on top of. By definition, "popular" music has the broadest appeal, which also means it's less complex. The more complex music is, the smaller the group that can appreciate it. I like to use Miles Davis' "Kind of Blue" as an example - I have musical training and still find it challenging to "grok".

So Michael and family had an unusual ability to change going forward, and keep their music engaging, while still having a broad appeal. Few musicians can do this, let alone to Michael's level.