(This seems a bit silly - why not always use the internal clock when programming? That way it can never fail...)
Its probably that the relevant parts of the chip (clock source select and ICSP programming) aren't neighbours so
you'd have to route a signal all the way across the die to implement it (and this wasn't deemed a priority as the
chip comes from the factory with internal 8MHz clock pre-fused - the common case is programming the chip
once from factory-state during mass-production)