Why is it that there are only three naturally occurring radioactive decay series now a days?

Answer 1

There are only three naturally occurring radioactive decay series—uranium, thorium, and actinium—because these series originate from long-lived parent isotopes that decay into a sequence of shorter-lived isotopes. Over geological timescales, most other isotopes have either decayed away or become stable, leaving these three series as the only ones with significant amounts of parent isotopes still present in nature. Additionally, these series are self-sustaining, as their decay products can also be radioactive, continuing the cycle. This results in a limited number of stable decay chains observable today.