There's a more pressing military goal for the Syrian government than Islamic State. First they need to sort things out with the Islamic Front. They're the amalgamation of some six Islamist military groups that occupy Idlib province, west of Aleppo.

The Islamic Front is Islamic State-Lite. Both groups want to destroy the Syrian government and establish a Salafist Sunni state under sharia law. The only difference between Islamic Front and Islamic State is that so far, the former hasn't indulged in attacks in Western nations. Therefore, they're not on designated terrorist lists. However, the huge loss of life in the Syrian civil war isn't because groups like Islamic Front were showing restraint. The Syrian government refers to them as terrorists, and so would any other government that had to deal with them.