Putting the cab at the front makes the most sense for visibility and driver experience, but the way most steam locomotives worked is by humans scooping solid fuel out of a hopper car behind the locomotive called a tender, into a furnace located in the cab of the locomotive.

If you put the cab at the front, getting the solid fuel to the furnace becomes a more complex operation since the cab and the tender are now separated by the length of the engine. Once liquid fuels became a thing and the need for shoveling solid fuel waned, the cab could be moved to the front much more easily.