Because it was not related to Covid. If someone with heart problems is admitted with Covid they will receive their heart medications in the hospital, that doesn't mean that now heart medication is a Covid medication, so doesn't get mentioned in studies. The same goes for dewormer, it was in normal use against worms. Only after people started believing that it could be a treatment for Covid, that it started being studied for that (with different protocols). And then we found out that it is just a dewormer like we thought in the beginning.