That's a little like asking, "Why make a role-playing game?".
(at least a traditional one)
I don't really want a return to 90s game design where what the author presents the game as being about one thing but then has rules for supporting other stuff and nothing for what the game is actually about.
Rules are useful and following the procedures of a game produces actual consistent play results.
The whole point of game design is to make a game that does something to help produce a desired type of play.
So either Mearls can design (which I think is the case) and the whole "DM empowerment" and "rulings rather than rules" talk in that article was just marketing to old schoolers, or he really intends for the rules not to matter nor to support the play of multiple edition users with rules modules.
I think it was marketing fluff to get old schoolers to think "he finally gets it" but I'm still waiting for anything to prove that modules that truly support different play styles will actually materialize. Right now we have what? Drop themes and backgrounds? And a tweet about healing rates?
That's pretty poor considering all the hype about "reunification."
Friends & Family playtest should have sorted the core system so the design team could have lead with tightly designed modules to give the various types of gaming experiences people want.
Modularity to produce specific results to support specific modes of play is simply not compatible with "rulings not rules" and "all the rules are just guidelines that the DM can use or ignore at any time." The DM empowerment play style is just one module/mode of many that are needed.