Here's why:

* No Atmosphere: Mercury has a very thin, negligible atmosphere, practically a vacuum. This means there's no significant amount of gases to condense and form clouds or rain.

* Extreme Temperature Fluctuations: Mercury's surface temperature swings wildly, from scorching hot during the day (over 400°C) to freezing cold at night (-170°C). This temperature range makes it impossible for any form of precipitation to exist.

In short, Mercury's lack of a substantial atmosphere and its extreme temperature variations preclude any form of precipitation.