Because it's just promoting the same gameplay - farming MOS as fast as you can for legendary drop chances / coins.
If the way you get legendaries is a problem, the solution isn't to make another way of getting them that's identical to the first, especially when it's just slapped ontop of an old system that people might have 100s of coins stockpiled or might have none at all after spending them. Making this a new system with a new currency would still be bad, co-opting the mysterious coin one is dreadfully worse for zero reason.