Several reasons.
It was a government garrison. Enemies are enemies.
This bunch was close to a major ISIS controlled population center. So logistics of concentrating sufficient numbers to overrun the garrison was probably a major factor.
Arms and ammo. The Syrians have huge stockpiles and they are probably kept at these undermanned bases. That’s a treasure for ISIS. I expect the Syrians haven’t got the manpower even to guard it much less to destroy it all. 170 dead out of that garrison, most likely mostly executed prisoners, suggests that enormous base was defended by less than 1000 men.
A runway is a runway. If its in reasonably good shape its an asset for resupply from the ISIS sponsors (Turkey?) or from the international black market. Useful to have such a thing right by one of the major population centers.
But why didn’t this runway/armory/base have air power to defend it?