They're not in the same orbital plane
For Iridium info, go here.
There are 66 active satellites and 6 on orbit spares. They are in 6 orbital planes, each inclined 86.4 degrees to the equator at 485 miles high orbits. Each plane is separated by 60 degrees. You can see a graphic representation here. A Java applet that lets you look at the constellation from different angles is available here.
If you were near the poles, you'd see an Iridium satellite passing overhead very often. At other places, not so often.