'cos it's a non-standard conversion. And, to answer your other point, if you lose one heater plug in the old series wired type, then yes, you do lose the lot, but it's so easy to find which one, short it out and use them for a shorter time. Better than losing them one at a time parallel wired and wondering why it doesn't start so well because you don't notice one or two going down until you do need them in a really cold spell. :-)