Calibri was supposedly easier to read by people with disabilities. While this itself is debatable, that's not the reasoning behind the font switch. The mere attempt at making life easier for disadvantaged people is labeled DEI and as such cannot be tolerated by this administration.
More charitably, the signaling could be: “keep the government as small as possible, but no smaller than that”, i.e. use things that basically mostly work and quit expending resources addressing every edge case, particularly when it’s performative (slight font variations) rather than obvious (a ramp to get into a public building)