I found this explanation about why nuns’ ordination can’t happen any longer. It discusses the flaw in the argument that the Buddha would have explicitly rescinded ordination by monks if he’d wanted to make unilateral ordination impossible. I haven’t yet come across a convincing rebuttal. Does anyone know if one exists?
"It has been argued that because the original allowance for bhikkhus to ordain bhikkhunīs was never explicitly rescinded, it is still in place, so bhikkhus may ordain bhikkhunīs without the candidates’ having to go through the preliminary procedures. This argument is based on drawing a parallel to the way in which the Acceptance of bhikkhus changed in the early years of the Teaching…
However, the two cases are not really parallel. In the case of bhikkhu ordination, when the Buddha allowed groups of bhikkhus to give the Going-forth and Acceptance by means of the three goings for refuge, he spelled out the procedure for them to follow. Once this procedure had been explicitly allowed, he had to explicitly rescind it when he set out a new procedure in Mv.I.28.3-5. In the case of bhikkhunī ordination, though, he did not explicitly spell out a procedure for the bhikkhus to follow when he allowed them of give full Acceptance to bhikkhunīs in Cv.X.2.1. Because the only procedure available to the bhikkhus at that time was the procedure for bhikkhu ordination, it was apparently understood that this would be the fallback procedure to press into service until the Bhikkhunī Saṅgha was well established and could play a role in the double ordination, involving both Saṅghas, stipulated in the sixth rule of respect.
This means that the allowance in Cv.X.2.1 is incomplete. It doesn’t follow the pattern in other, similar allowances, where, once the Buddha has given the allowance, he immediately provides instructions for how to carry it out. At the same time, it only partially falls in line with the sixth rule of respect. In this sense, the allowance for double ordination given in Cv.X.17.2-8 simply completes the earlier, incomplete allowance. It doesn’t rescind the earlier allowance, because there is nothing in the earlier allowance that needs to be rescinded. Bhikkhus still give full Acceptance to bhikkhunīs, simply that they have to do so in line with the new procedures, which are fully in line with the sixth rule of respect.
Once these new procedures were spelled out, the earlier fallback procedure could no longer be used for that purpose from that time onward, as that would violate the new allowance."