The natural progression after McGregor beat Poirier was to have the Irish fighter take on one of the division's best grapplers - Mendes, Frankie, or even Lentz. Siver was almost a step back for McGregor, even though the German lasted longer than Dustin.
I think the UFC brass recognizes McGregor is promotion gold. He's really the first guy to come along since BJ Penn who has the ability to generate serious fan interest in the lighter divisions. That's something that even stars like Aldo, Frankie, Mighty Mouse, and Urijah have all failed to do. And so they didn't want one of the division's best grapplers to grind out a victory against McGregor and strangle their golden goose before he had a chance to lay a few golden eggs.
Yeah, just like Hominick, KZ, Lamas, Edgar and Florian had to, it's only fair.
Oh... wait.