If eukaryotes can form multicellular bodies, why are all eukaryotes not multicellular? Eg, protists.

If vertebrates can evolve flight, why are all vertebrates not flying?

Same question. Just because an organism has the potential to evolve in a certain way doesn't mean that it will: it will evolve in response to the specific pressures exerted by its environment. For many unicellular eukaryotes, their present form is nicely optimised for their evironment, and there would be no benefit to multicellularity.