Someone did post this earlier which goes a little further sourcing wise:
https://www.businessinsider.com/acting-pentagon-chief-halts-cooperation-with-biden-team-on-transition-2020-12
From the article;
Acting Defense Secretary Chris Miller sent an order instructing staff to cancel scheduled transition meetings Thursday night, Axios reported Friday.
A defense official told Pentagon press in an emailed statement that the meetings were canceled due to competing priorities. The meetings, the official explained, are expected to be rescheduled for after the holidays, a two-week breakagreed to by both the Pentagon and the Biden transition team.
>>12079844 (pb)