For a long time ISIS seemed to be avoiding antagonizing Turkey and Erdogan didn't seem to mind his country's own extremists going off to die in Syria. That seems to have changed recently. Further, Turkey most certainly sees a military engagement in Syria as a way to extend its influence further south on the peninsula. Nobody wants to pay the actual material costs of occupying ISIS territory but Turkey is likely the best combination of seeing some upside in making the effort and having the capability to do so. Saudi Arabia and the Gulf states have been involved in the conflict too but their armies are not nearly as capable.

It's also entirely possible that Erdogan knew he had enemies in the military and was waiting until after what he may have seen as an inevitable purge. The military certainly lost a lot of experience and going to war is one way to get the new commanders up to snuff. Syria is no joke but it's low stakes enough that some setbacks wouldn't be catastrophic for a country as powerful as Turkey. It may not have been in Turkey's best interests to wait but it could certainly have been in Erdogan's best interests.