The Moon acts as a physical shield that isolates a far-side lunar-surface telescope from radio interference from sources on the Earth’s surface, the ionosphere, Earth-orbiting satellites, and the Sun’s radio emission during the lunar night.
I’m also not sure it would be that much more effort. Economically speaking, most of the cost probably comes from rocket launches, which are roughly similar for Moon missions and other far-out space missions. Construction on the Moon has unique challenges (sharp regolith, temperatures), but you get a supporting base for free. If you had to supply and launch support structures, it might be more costly overal.