Because I wanted the 6.2 but I didn't feel like spending the $10k or so more for just a comprable base esky, and I am not a fan of either the Esky front grille nor of the interior layout ... plus I enjoy the modding.
Not sure why you would ask though, changing the tails and third to that of the esky is a rather common modification with both Tahoe and Yukon owners, whom I'm sure all have their own valid reasons for not purchasing an Escalade.