Here's why:

* Gene flow refers to the movement of alleles (gene variants) between populations.

* If there's gene flow, it can introduce new alleles or change the frequency of existing alleles in a population, disrupting the equilibrium.

* Hardy-Weinberg equilibrium assumes that the population is isolated and not exchanging genetic material with other populations.

Let me know if you'd like to know about other conditions for Hardy-Weinberg equilibrium!