Well, if they want to make Civ6 better than Civ5, they won't want to revert back to mere 2D leaders for scenarios after so much work with the base game leaders? In addition: That way they demonstrate what we can expect in the DLCs to come (good and subtle advertisement strategy). If you look at the potential for DLCs, even the "tack on stuff" must be interesting enough for players to buy it. If leaders are just in 2D, the interest will likely be much smaller. And if they introduce all these leaders into the vanilla game as well, they will give that an additional flair with more leaders to choose from. So they got two birds with one stone.
Moreover, it would be a logical explanation for why they have provided additional leaders for certain civs but not for others. So they won't have to fear being critizised for being politically incorrect (we see this already in other threads with regard to the included civs).
Finally, all the history based arguments - while definitely worthwhile to read - are way to sophisticated IMHO. After all they are supposed to make money with this game (which they should after all the effort they have put into it).
