Dhristadyumna was the crown prince and C-in-C of the Panchala forces, the sworn rival of the Kurus. The Pandavas did not have any blood relatives fight for them. Their own inheritance, i.e. the Kuru war-machine (Bheeshma, Drona, Ashwattama, Kripacharya etc and their thrall kingdoms) was their main opponent. Shalya and the Madras and the Yadava army, blood all of them were also against them. For these veritable paupers, only the marriage relations stood with the Pandavas.... viz. Panchalas, Matsya and Kashi. Strangely, all these kingdoms had old axes to grind with Kurus too.

Out of these allies the Panchalas were the strongest .... it's logical that the crown prince of Panchalas, who was an Athiratha himseld, be the C-in-C. Wonder what pound of flesh they would have demanded if they survived the war..... would Yudhishtira salami slice Kuru holdings as war reparations?
PS: Also, it might be the other funda a-la "Why was Sachin not always our captain. He is the most powerful warrior, no?"
PPS: Bhima proposed Shikhandi as the C-in-C. I wonder why. There are explanations in Randam Oozham and other works (Shikhandi was one of those strange, quiet, thin and effete psychos. A Chuppa Rustom single minded killer who Bhima recognized.... after all, he was highly under-estimated himself)