It is not incompatible with all road bikes, but with compact ones (i.e. most modern-geometry road bikes). The reasons for incompatibility are stated
here.
From what I have seen, the major problem lies with the part of the frame that attaches to the bike.
– There needs to be room around the bottom bracket to attach the attachment bracket (may be a problem on a bike that has really short chainstays).
– The frame has to be far enough in front of the bottom bracket to clear the foot and pedal.
– The front triangle has to be long enough to allow proper steering of the bike.