Basically it's been that there had been no studies to prove it works for covid. It's been used mostly in poorer countries that have had it on hand because it's relatively inexpensive. Doctors in those countries have been sharing info with each other and that's how it started being used a lot.

During that time gather data for this use was begun and it's taken this long to gather enough, over enough time, to prove its efficacy according to the standards required.

Studies have been done and show that it is effective: https://covid19criticalcare.com/

Maybe because you cannot use it as a prophylactic, so it doesn't threaten EUA for vaccines?