If you think about why a vacuum flask or thermos is used to keep drinks hot (or cold) for an amount of time you'll realise that space / a vacuum is a very good

If you think about why a vacuum flask or thermos is used to keep drinks hot (or cold) for an amount of time you'll realise that space / a vacuum is a very good insulator. An object in space loses heat according to the Stefan-Boltzmann law of radiation. That use the fourth power of the temperature differential so you get boost from the background of space being -270C. BUT the Stefan-Boltzmann constant is 5.67 × 10−8 J/s · m2 · K4 and that is the problem. You can do the numbers yourself but various internet sources suggest that spacecraft radiators can only cool between 100 and 350 W of internally generated heat per square meter. That is a big radiator for not much cooling.