Why is it difficult for astronomers to detect groups of planets similiar to our own solar system?

First, nobody knows the first fact on which to base any such belief. They GUESS they are common. It looks probable, since everything we can see has something orbiting around it, but still they don't know any such thing.

Second, a planet is awfully small and dim compared to a sun, and they are closer together than most telescopes can resolve. So they have to use technical trickery to detect anything, and they mostly only detect huge planets that block some portion of the star when they pass in front of it. A planet the size of Earth blocks only 1/10,000 of the sun's light for a moment once every year, so that gives an idea how hard it would be to see from some distance.