Flybe simply want to fill their aircraft. Whether that's SQ connections at MAN, DUS or CDG is neither here nor there. Routes to the likes of DUS or CDG from EDI or GLA will not be filled with MAN connecting traffic.
MAN does not have daily SQ flights in both directions, all of which are in the morning. BE would want to give their customers choice. Unless or until MAN gains multiple daily flights from SQ they will spread their net accordingly.