It's complicated. Officially, the reason is that the bhiksuni pratimoksa lineage in the Theravada and Mahasarvastivada vinayas died out. A bhiksuni can only be brought in by a congregation of bhiksunis and a bhiksu can only be brought in by a congregation of bhiksus. Therefore, there can be no more bhiksunis in those vinaya traditions. The Dharmaguptaka vinaya still has a thriving lineage of bhiksunis and there are many nuns in east Asia.

There have been attempts to revive the bhiksuni lineage in Theravada, which has been subject to lively debate whether it is in accordance with the vinaya code. I don't recall the details, but in one location they have done it, and it is met with mixed reception. They are not recognized by the major monasteries or by the government, which creates difficulties because the government provides some regulation of monastics in these countries.