Because the US have a rather good track record concerning internet gouvernance. By contrast the main drivers for ITU involvement are gouverments with rather dubious motivations, like Russia.

So as much as I would like to see a more international internet gouvernance, I am afraid that the ITU ( with their "old men with pens" influence) would most likely not do a very good job compared to ICANN and W3C. (And besides, never change a running system ;)