Several reasons:
- Cormier still has 25 pounds to go and he's not risking a big water cut. So he needs more time to diet steadily;
- Cormier needs to build his name with UFC fans. The only good LHW currently unscheduled is Gus, and, although he's good, beating Frank Mir gives you much more name value and exposure among casual fans;
- Cormier wanted to fight 5 times in 2013, so he may very well fight Mir in April, fight a LHW contender in summer and fight LHW champ late in Autumn/early winter. If he stays healthy and wins without taking much damage it is possible.
So, in the perfect universe where Cormier fights 4 times a year and Jones has 3 defences, everything is fine.