What you’re seeing is usually a local optimum: one aggressive/spiky strategy found something that works and crowded out everything else, so diversity collapses.
There are some things that can break it: Just waiting. At this stage you often get crashes from starvation, pathogens or reproduction overshoot. Seasons and climate cycles can shake dominant strategies loose: https://soupof.life/concepts/world-systems/climate-seasons and low-diversity phases like this are often fragile. With a bit of bad luck, this can turn into a big reset or even a near-extinction.
There’s no manual intervention by design, so it’s mostly patience or starting a fresh world. But in my experience, long stagnant plateaus are often right before something dramatic happens.