Having more than one exam board results in competition, which is a good thing if you are buying computers, washing up liquid, or waterproof gloves; but a bad thing if you are buying GCSE courses because the only way the they can compete is by being easier than the opposition. This obviously results in a race to the bottom thus:
"Our Science course is quite demanding and gets the kids a GCSE"
"Our Science course is so easy that a performing chimp would scoff at its simplicity. In fact, last year one of the carrots from my garden achieved a GCSE grade "C" and would have gone on to take our 'A' Level had it not accidentally been eaten by Harry, our pet Labrador.
"Great Salesman B, your course sounds ideal for our students. We'll take it."
This is why nobody outside schools has any faith whatsoever in the exam system any more.