Flowering plants dominated when they showed up on the evolutionary scene, partially because it's faster to speciate and get new diversity.

For plants that pollinate via dispersal of pollen in the wind, when any pollen lands on any plant it pulls it back to the average.

The unrestricted gene flow prevents reproductive isolation necessary for speciation.

Versus flowering plants can coevolve with specific species of pollinators allowing them to be physically adjacent but still isolated, allowing faster speciation.

Faster speciation in niches leads to faster OODA loops.

Faster speciation allows not a broad resiliently-good-enough species but a lot of ones that hyper specialize to niches, and give more genetic diversity in the ecosystem.

An individual type of flowering plant will be less common, but the class of flowering plants will be much more common than any individual non-flowering plant.

The class has diversity that the species can't.

One measure of health in an ecosystem is the amount of genetic diversity.

The more diversity, the more likely the overall ecosystem is to have an "answer" to a new problem that pops up.