Well, Galileo hadn't perfected his best telescope yet.
Galileo discovered the biggest four moons (the Galilean Moons). A fifth moon was not discovered until the end of the 19th Century. By the 1970s, there were only a dozen known moons. By the end of the 20th Century, there were only 15 or 16 known moons. The number has grown exponentially since then. Most of these are just big chunks of ice or rock.