When light travels, it spreads out as it goes, making it less powerful for a given area. This is the famous inverse square law.

Radar decreases the same way. However, radar is even worse in that it also decreases on the way back to the sender.

So if you double the distance to the target, the radar signal coming back is 1/16 as strong. If you increase the distance by a factor of 1,000, then the signal coming back is 1/1,000,000,000,000 as strong. And so on...

This makes radar a pretty poor tool for space exploration.