Mxsmanic wrote:
From what I understand, icing protection should be turned on if the
outside temperature is 5° C or less. However, what is the reason for
not having it on all the time?

Some ice protection equipment uses a fluid that is of limited quantity
on-board, not to mention that it costs money. Other ice protection
devices can put a drain on a smaller aircraft's power. Turbine drivers
might chime in with their reasons, I don't know them.

Ice protection isn't always needed when it's cold, you also need
moisture to create ice.