I have a guesthouse and I have a sign saying "available room". Someone told me better to say "room available" Why is this?
This source makes a good classification of adjectives into attributive (premodifying the noun), postpositive (postmodifying the noun), and predicative (those which modify the subject being inside the predicate).
"available" is mentioned as a typically postpositive adjective and, in line with one of the comments above, it is used in that position because it is merely more idiomatic (the source says that postpositive adjectives "are normally found in a number of fixed expressions"). The adjective "available" is in fact usually found after the noun to which it refers: We have a room available / There is a room available.