Because they weren't and he wasn't. Only the Ustase (& it's modern-day fan club), who were the real German Nazi boot-lickers unsuccessfully try to slime Mihailovic with that in order to try and make themselves look less dirty.
Okay, where would you like to begin? With Pecanac’s Chetniks of Sumadija? With Dangic’s Chetniks of Eastern Bosnia? With Jevdjevic’s Chetniks of Hercegovina? With Djujic’s Chetniks of Dalmatia and Lika? With Djukanovic’s Chetniks of Western Montenegro? With Djurisic’s Chetniks of Eastern Montenegro and the Sandzak? Mihailovic’s Chetniks roughly accounted for less than 10% of all Serbian Chetniks during WW2, and even they took arms from the Germans to fight Germany’s enemies. All the ones I listed above openly sided with the Axis and formed the majority of Serbian Chetniks in WW2.