Because every once in awhile promoters aren't complete morons and read the writing on the wall. Everyone knew from back-room scouting that Cain, Weidman, Khabib, and Jones were monsters and would wreck their way to the top.
All of them did without exception, with varying levels of promotion over time. Khabib was stiffed because they had chosen a golden boy for his division, Weidman almost missed his shot because Anderson was using his leverage not to fight him, Jones was fast-tracked in the promotion sense and his personal shit ended up biting them, Cain was in a shallow division and then usurped the spot from the golden boy Brock so UFC had to fall in line.