The atmosphere is really good at distorting optical signals. It's not as good at distorting radio waves.

Additionally, radio telescopes tend to be enormous - much bigger then optical telescopes. It's one thing to launch a bus-sized instrument into orbit. It's another to launch a stadium-sized instrument into orbit.

Optical telescopes are in space to avoid the effect of the atmosphere. Radio telescopes aren't effected in the same way so I don't think there would be an advantage for them to be in orbit and it would cost a lot more.