Maybe because that would be "Ist das weit?" Slight difference in English in the meaning. "It" is very general. "That" is almost like pointing at something, although in a general context when the object is not present, it would be referring to a specific object just referred to. I wonder if they accept "Is that far?"
In talking about distance, you can say "Is it a long way?" or "Is it far?". But, generally "Is it long?" would refer to the length of a table or a stick or something more tangible, unless you added the word "a" before it and something like "distance", "time", "way", after it. Somehow by using those words it makes "long" okay.