Why are radioactive isotopes often called "geological clocks"?

Answer 1

Because they decay over time at a known rate. You can date things by them and tell exactly how old they are by the amount of radioactive decay.

Answer 2

It is due to the 'half life' of radioactive isotopes. They decay at a consistent and measurable rate. What's more interesting is that an isotope either will or will not decay. It's a fifty - fifty chance and scientists have yet to determine if a given radioactive molecule will decay or not.