I can't explain why Covid specifically is comparatively easy to make a vaccine for, at least compared to a cold. Party of it, presumably, is the incentive to make one: a disease that has the potential to kill so many is far more of a threat than a disease that has the potential to make people miserable for a few days, and has prompted one of the most intense medical research programmes in history.
Also, there is no single "cold" virus, but a couple of hundred of them: you'd have to make a vaccine for each one, or a vaccine in combination. Also, the mutation rate is faster than a vaccine can keep up with.
This presumably also explains why flu vaccine is both important and also annual: the virus also mutates, so no one vaccine would be useful for long, but since flu actually *does* have the potential to kill, it's that much more important to have a vaccine for it.