Universal Pictures bought the rights to make Lego movies, but that didn't include the rights to any of the popular characters from The Lego Movie or its spin-offs. Those stayed with Warner Brothers (even though WB can no longer use Lego, so those characters are functionally dead).

Universal also didn't get the rights to any of the franchise characters typically associated with Lego, so they can't use Boba Fett, or Batman, or Gandalf, or Harry Potter, or even Jack Sparrow.

So they're sitting on this huge IP and have no way to actually use it. And so Pharrell comes along and says he wants to make a documentary about himself, and they say nah that's boring, nobody wants to see that. Then someone realizes they're going to lose the rights to Lego if they don't make a movie with them soon. And here we are.