Final answer:
A site might be open only part of the year due to seasonal weather conditions affecting its operations or due to government regulations which may dictate its accessibility.
Step-by-step explanation:
The question of why a site might only be open for a portion of the year can have various answers depending on the context. Based on the provided information, it seems that seasonal weather conditions play a significant role in determining when a site can be open. For example, at locations ideal for astronomical observations, the weather can dictate site accessibility, with clear weather present up to 75% of the time. Elsewhere, government regulations, including zoning laws and business operation restrictions, could define the hours or months during which a site may operate.