Boy...you know your priorities when you leave your tractor inside, and a brand new truck outside!!!
Well, I do the same. The 790 (and my prior tractor, a 670) were stored in the garage.
I don't see any reason you couldn't park your Kubota outside. Other then the paint fading from the sun...well, these machines are frequently left outside for months at a time at the dealer. A tarp should reduce most paint fading...but make sure there's enough air moving under it to prevent condensation.
I park mine inside to reduce any theft potential. Also, since I don't have a block heater, it's easier to start (garage is in an unheated part of the house...but still warm enough).
I'll probably go for the block heater this year though, so I can leave the tractor out for snow removal (not a lot of clearance under the garage door. Any accumulation of snow might make it tough to get out...wouldn't that be a drag!!!).
So, your truck undoubtably cost more then your Kubota. Your tractor is...just that, a tractor. If where you park it is safe from prying eyes, it would be safe to leave it outside. They're built to be outside (I can't believe how much some of us...including me...baby our equipment!). If your tractor doesn't have a block heater...I'd sure consider getting one.