It has to do mainly with the tenacity of the defenders. You see, you would not expect good treatment if you were captured by ISIS, so you're truly fighting for your life. A similar situation was seen at
Kuweires Air Base, whose siege was lifted by the Tiger Forces on November 10 2015.
Many also praise DeZ Garrison Commander, Issam Zahreddine, for good leadership and for being in contact with his own troops all the time, even personally leading battles (as seen in some videos).
Also a key factor is that the Syrians have complete air dominance, obviously, and we can add the Russian support as well, in form of more precise airstrikes and airdrops for the besieged soldiers.
The situation went from being relatively stable to shitty when the US-led Coalition "Accidentally" bombed SAA positions and killed a number of them, almost an instant later ISIS launched an attack and managed to seize more territory, even cutting the SAA forces in half, as it stays right now.
DeZ has been a pain in the ass for the ISIS war effort, it was always there, disrupting ISIS supply lines and draining resources that could be best used elsewhere. If not for the DeZ effort, ISIS could have made life even more complicated for the Iraqis and Kurds in Mosul or anybody else in their battles, SDF in Raqqah, SAA driving ISIS out of Aleppo Province, Palmyra or the Iraqi effort to push ISIS out of Fallujah and Ramadi.
