He, or the UFC, will say it's because Khabib isn't a draw and won't bring in more money, but the UFC knows KM is a nightmare matchup for McGregor and won't take the chance of someone beating CM oin short notice.
The UFC could spin a short notice loss easier than a full camp loss, it doesn't make sense to me, because if CM beat KM, even the most ferverent doubters would have to accept that he is one of the top fighters on the planet. But the UFC will not let that happen, they want a fan friendly striking war, not a gritty, tough grappling centric fight.