ELI5: Why is mercury the only metal that exists as a liquid at room temperature and pressure?

This question has probably been asked a thousand times before, but I haven’t been able to get a proper answer without diving into relativistic effects and electronic configurations.

What are these effects caused by? What exactly is the process that leads to this? I know some other elements like Gallium can exist as a liquid at \~30 degrees, but that has a different reasoning to Mercury’s case.