Why is it that only Promethium (Pm) is the only radioactive lanthanide?

I was trying to figure out what percentage of all radioactive elements in the Periodic Table the f-block accounts for, when it struck me that Promethium was the only radioactive lanthanide shown. I found this a bit odd, since I expected to see a couple of other radioactive elements there as well, especially since Promethium is (somewhat) near the middle of the series.