Liquid evaporates in sunlight. It doesn't re-condense if the seal on the container isn't good enough to contain it. This, however, is not the real question or answer to why those potions that evaporate on their own do so.

Clearly, if a potion evaporates after a time limit, it's not meant to be something you can keep forever. So, if you're to get a message telling you the potion is gone or no good any more, what would you rather have? Evaporating is as good a message as anything. I'd prefer that to "Your potion has lost its potency and vanished."

It may not be all that realistic, but if the potion is designed to go away, at least it tells you when it happens.