Because there is not enough room to install a new sensor to a certain spot that can read the rotor. The only option would be to add a new piece to the rotor and that causes a series of other problems, it is much better to just convert the signal as MisterBill2 said.So why not get a Hall sensor as your pick up? Get one made for acting with a single polarity (Ithink they most commonly use magnetic south, but may be wrong) This would allow you to use the existing magnets in the flywheel to get a DC pulse to the ignition rather than the AC pulse that it normally makes. They are normally AC because the magnets in the flywheel has both N and S poles, and that generates the AC output.