Here's why:

* Star Formation: Star clusters are born from giant clouds of gas and dust. The stars within a cluster form at roughly the same time.

* Massive Stars: Supernovae are caused by the explosive deaths of massive stars (at least 8 times the mass of our sun). These massive stars have very short lifespans, often burning through their fuel in just a few million years.

* Early Explosion: Because massive stars die quickly, most of the supernovae in a star cluster will occur in the first 100 million years. After this period, most of the massive stars will have already exploded.

However, there are exceptions:

* Intermediate-Mass Stars: Stars between 8 and 20 solar masses can evolve into supernovae, but their lifespans are longer, lasting hundreds of millions of years. These supernovae may occur later in the cluster's history.

* Type Ia Supernovae: These supernovae are caused by the explosion of a white dwarf star that has accreted material from a companion star. These events can occur much later, even billions of years after the cluster's formation.

Overall: While the majority of supernovae in a star cluster happen within the first 100 million years, there are some exceptions, and supernovae can occur throughout the lifetime of the cluster, although at a decreasing rate.