It has always been possible to instruct on a PPL, but for the JAA period such an instructor could not be remunerated. A FI needs CPL level knowledge and as the hours required to become a FI are less with a CPL it makes more sense to follow that route. A PPL FI can only teach PPL, but additionally, any rating they hold that can be added to a PPL and for which they are qualified to instruct. To teach for a CPL then the FI must have a CPL.