Because the behaviour was inherited from C, which uses explicit break instead. Switch fallthrough was much more useful then and that's why it was chosen as the "default" behaviour.
There was just a lot more programmer time and a lot less machine time so designing for maximum potential efficiency instead of readability made a lot more sense. And compilers had a lot less power to optimize (in many respects). You can look at things like Duff's Device for examples of how this behaviour was used.
