[masraum]

What he said. Astronomy is all about catching very faint light. You are trying to gather more light and focus it into your eye. The bigger the telescope, the more light you get. My old scope was an 8" Newtonian tube on a dobsonian mount (about the lightest and simplest of mounts). The tube was 4' long and a little over 8" around. I think the whole thing weighed 40ish pounds. It's also a somewhat delicate piece of equipment, so you don't want to knock it around a bunch (the lenses have to be in alignment, and can be knocked out).

It's not unusual for folks to have 12" or 18" tubes (vs my old 8"). The weight can really start to add up, and even if it's not that heavy, it's large. If you aren't using a dobsonian mount, but you are using a GEM (German Equatorial Mount) which has large legs, large counterweights and a tracking motor, then the mount is probably the bulk of the weight. Then there's the act of having to set those things up (align them) every time you take them out so their tracking is accurate.

By having an observatory, everything is sitting there and ready to go (and at the ambient temp).

Yeah, things need to be at ambient temp. If your scope/mirror/lenses are different than ambient temp, then you don't get a good image and you can get condensation (which can happen at ambient temp too if it's late and the dew has started). Think about the heat rising from a hot road in the summer and how looking through that makes things shimmer/wavy when looking through the hot air. Having aspects of your scope at different temperatures makes the same thing happen on a much smaller scale, but when you're trying to see something that's really faint and you've just catching a few photons of light here and there, then that can cause an issue.