Why is heating radiators filled with water?

Because water is widespread everywhere, relatively cheap, has good thermal conductivity, heat transfer and heat capacity, chemically little aggressive to the inner wall of heating radiators. It optimally combines all the above qualities as a heat carrier, and therefore is exclusively used in heating systems (both in centralized public and private). There are liquids with better properties, but they are usually much more expensive and economically unreasonable to use them.