Why might a planet situated mathematically in the middle of the HZ still be functionally dry?

Answer

It might have low enough atmospheric pressure for water to sublimate or boil off rapidly

A very thin atmosphere results in insufficient pressure, allowing water to transition directly from ice to gas (sublimate) or boil off, despite being at the right orbital distance.

Why might a planet situated mathematically in the middle of the HZ still be functionally dry?
astronomyplanetssolar systemhabitable zonegoldilocks zone