why is it su instead of sulla?
'Cause we are talking about a "generic" hill. It is on A hill. If you want to say "It is on THE hill", so you'd say "È sulla collina."
Is there no shorter way to write this sentence? For example if it were 'the hill' it would be simply 'sulla collina'. Is there no shorter or less awkward way to write 'su una'?
Why isn't "The hotel is up a hill" also acceptable?
I couldn't get a "correct" until I ran "su" and "una" together, while separating everything else orally: "L'albergo - è - su'una - collina"
Do you mean in a Say what you hear? Because that would be incorrect if written
Why "uphill" isn't right?
Because uphill and "on a hill" don't mean the same thing. One refers to a direction, the other to a location.