Why is a small world like Mercury unsuitable for maintaining surface liquid water over billions of years?

Its small mass prevents it from holding onto a substantial atmosphere over long periods

While orbital position dictates the potential for liquid water based on temperature, the long-term persistence of life requires atmospheric stability, which is intrinsically linked to planetary mass and gravity. A planet must retain its atmosphere over billions of years for evolution to occur. Mercury, being a small world, lacks the necessary gravitational pull to hold onto a substantial atmosphere over cosmological timescales. Without this necessary atmospheric blanket, surface conditions fluctuate wildly, and any water vapor would likely be stripped away by stellar winds, making long-term habitability impossible regardless of its instantaneous temperature relative to the HZ.