For a number of reasons, one of which is that space still isn’t cold enough! Certain satellites often need cooling in space to work properly, the Herschel Space Telescope (pictured above) is a prime example of this. This telescope detects infrared radiation. This type of radiation is associated with heat so any sources of heat can be detected.
It is often said that space is very cold and this means that the concept of needing to cool something in space seems bizarre. The issue originates from two main reasons. The first is that space is practically empty so as heat radiation travels it doesn’t disperse much and heats the first thing it comes in contact with. This is how the Sun can warm up the Earth despite being such a long way away. Satellites that need to be cooled counteract this by using a sunshield, this can block the infrared radiation reaching the sensitive instruments.
The other problem is the electronics on board. Much of the circuitry onboard generates heat, so the satellite will slowly warm itself up during operation. If the lenses and sensors of a telescope are warm they create a lot of interference and noise when trying to detect faint signals. To prevent this many satellites are actively cooled, this keeps the equipment cool enough that it generates minimal infrared radiation. The ensures the maximum impact from any telescope.
Answered by Josh Barker from the National Space Centre
Image courtesy of ESA