They won't allow it because for the most part the show must go on and its only after the fights when it comes out when people pay tickets and hotel and travel fees who you think will get cheated most test aren't reveal until after the fight and yes there are countless of fighters who have tested postive.
One of the biggest areas of concern for boxing is this: There's no such thing as a free lunch. True, random, OSDT style drug testing costs money to implement and enforce. Someone's got to foot the bill for that, and there's few expenses that get added on to the sport that don't ultimately result in the fighter making less money for it. For monster fights like Pac-Floyd, it's no big deal to implement the testing. They can afford it. But if it is a sportwide requirement, we're going to see more mismatches and fewer decent small cards because the cost of testing would be a significant new expense. It's not cheap to do. With no central governing body, who eats the cost of it? Or, do we only do OSDT in "big" fights? Those are tough questions that need to be solved. I wish it were as simple to simply add it, but these things don't come free.