The crux of it is that the French Revolution didn't just change France, but the whole idea of how society should actually function - that was completely new. Before that, it was clear-cut: the king rules, the nobility has privileges, the church has power, end of story. The Revolution, on the other hand, introduced concepts like human rights, equality before the law, and popular sovereignty into the world, which then had ripple effects everywhere across Europe and later worldwide. Without this break with the absolutist order, there probably wouldn't have been modern democracies or all the constitutions we know today - that's why historians regard it as a turning point.