The freewheel hub was invented and introduced in 1898, and rear carrier racks followed soon after. So that became the logical location for luggage. However, prior to that, when bicycles were fixed wheel, the emergency exit route was via the rear, so it was not feasible to fit luggage behind the saddle. In that era, leather frame bags were sold as an accessory, and some manufacturers made front carrier racks with a leather or canvas luggage bag to suit.
Smaller tool bags were initially made to fit behind a saddle, on the top tube or to the handlebar, the saddle- mounted tool bag subsequently becoming the British cycle industry standard.