Thermal management is a huge issue for spacecraft. In atmosphere, the bulk of cooling for things like electronics would be convective, from transferring the heat into a fluid (air/water/etc) which then moves away with the heat. In space, you don’t have a fluid for convective cooling, so your cooling is all radiative - essentially just emitting infrared energy. This is far, far less efficient - you need much more material and surface area to get the same cooling.

Dark objects are better at radiative cooling… unfortunately, they’re also far better at absorbing radiative energy. Like the oodles of it coming out of the sun. That’s why dark objects are dark - they’re absorbing the energy. However, it also means that your thermal management is far more difficult because you’re absorbing a lot more heat. It can be worked around, but it makes the spacecraft larger and heavier, which is the antithesis of space work. So spacecraft have traditionally tried to reject as much absorbed energy as possible, which by definition makes them reflective.