Once upon a time the idea was that you could glide back onto the airfield from any point in the circuit; it is not a bad principal to adhere to now. If you can't reach the airfield your circuit is too big! At around the same time the current PPL syllabus was put together and consists of a series of building blocks which ideally should be conducted in the order designated.
The student is progressively taught up to Ex 12/13 where they pound the circuit until they can demonstrate 3 consecutive safe circuits before you let them go solo. The solo is therefore a repeat of what they have already demonstrated they can handle. As a precaution we demonstrate glide and flapless approaches so they can see what to do if the unlikely happened. They also have to see and demonstrate EFATO so they are fairly well prepared when they embark on their first solo.
The PFL is probably the most demanding exercise on the PPL syllabus and usually takes several goes to get close to doing even a reasonable one. It demands more skill and poses its own dangers for a low houred student. There is absolutely no point in introducing this until the student has mastered landing correctly and consolidated those skills especially as they will only be operating at circuit height. The PFL is taught prior to out of circuit solo so that the student can demonstrate the ability to deal with an out of circuit emergency and is another reason why a FI has to be unrestricted to authorise the first out of circuit solo.
On the microlight syllabus they teach PFLs pre-solo for a very different reason; the aircraft all began with notoriously unreliable engines and their glide performance means they have little chance of getting back to the airfield. Therefore with a high probability of a field landing it was introduced early but that really is not applicable to a certified SEP aeroplane.