Almost certainly French thread and tubing diameters, but a previous owner has already found a replacement stem that fits (physically, if not aesthetically). If you really want to go threadless (but why?), you'd need a new fork which would obviate the French thread issue. If the current fork/headset/stem/etc are ok, why bother changing them out?

You'd likely need to do some surgery on the drive side rear dropout to get anything other than a Simplex derailleur to work -- keep that in mind if you're thinking of a drivetrain update. Again, it appears a previous owner has already updated the crank, likely by using a new spindle with the original French thread cups. Nothing wrong with that; if it works, don't fix it.