The Service Notice I saw never said why.
It occurrs to me though, Ford did a change from 5 to 6 quarts of oil, then later Ford issues the oil weight change. At the time I just thought that a lighter weight oil was a natural step when the 3V came out and both the tolerances got tighter than they already were on the 2v's and 4v's, and the oil driven cam phasing was incorporated. I just assumed that they issued the bulletin to prevent confusion over oil weights at service stations, DIY customers, and dealerships.
But then I got to thinking.... as well engineered as it is, if there is one glaring problem with the 4.6 block, it is that the oil drainback rate is less than stellar for the lubrication needs of the overhead cam architecture it supports. It's one of the major design elements they went out of their way to improve on the Coyote platform (probabily mostly due to the TIVCT architecture).
My first guess is that Ford did some special testing on the hush back in the day and concluded that a faster flowing oil coupled with the previous increase in oil capacity would keep the pickup tube from sucking air under the worst possible conditions (angle or slosh level).
Or maybe they were just doing too much warrantee work for cylinder rings and decided that a lighter weight oil was a better idea...
Again, it's all speculation on my part....just my best guess or two. There could be any number of reasons.