Telescopes don’t work when they’re not at the same temperature as the surrounding air. They don’t work if there is glass between them and the sky. (Okay, they work, just not very well.) So: you use them outside, and any space used to protect them from the elements is best if it’s as much like outside as possible.
Observatory domes, retractable roofs, etc.: all very fancy ways to keep the rain off.
Wildlife has a habit of getting into these spaces, especially the small critters. Spiderwebs are a frequent feature. Birds appreciate the shelter from the elements, and will happily build nests if given sufficient access. It’s amazing what they’ll squeeze through, if for no other reason than to leave a stark white splotch on an all-black telescope. At least it seems that way.
This doesn’t include the exterior-only wildlife population that brightens up the occasional observation session. Nocturnal critters get used to the quiet around daytime spaces, so it’s best to keep alert for opossums and skunks and other adorable visitors when out late at night. That black-and-white blur is probably just a feral cat, but you know what? Best not find out for sure.