The big thing standing in the way of a revised PHB is the PHBs still sitting around. If they were to put out a new book, not only are they spending the time/effort to do all the work on making the new book (formatting, editting, updating, art, etc), and the cost of making books (printing, shipping, etc). They are also essentially (no pun intended) giving up on ever selling more copies of the original PHB (and probably a few other books, like the PHB2 if they are taking many of the classes and races from that book ... and if they are including builds from Martial Power, Arcane Power and Primal Power? Say goodbye to those book sales as well). Creating a book that is mostly redundant AND up to date information would basically kill any potential to sell the older books. Any books that WOTC is still sitting on, they'd pretty much need to just destroy, since storing them would be more costly with no intent to sell. And any store holding on to those books would either send them back, or feel foolish trying to sell a product that is inferior to the new product, not just in how much content it has, but that it's out of date.
Ultimately, the books would only be a good idea if the potential revenue is enough to not just offset the cost of the book itself (Which is, say, how Paizo would have settled on their massive book) but ALSO have to factor in the extra costs involved with obsoleting a lot of their older books which they are probaby still sitting on. They aren't the movie business, able to release a barebones DVD, and follow it up with the extended cut a year later in the hopes of hitting the same people twice. Their audience is small enough where trying to sell the same thing repackaged is rarely going to work. (There are some exceptions, the Spell Compendium and Rules Compendium from 3.5, and in the latter case 4e as well, are examples where they take a portion of content from a lot of books and put them into a single book. Those were useful enough that even someone who had purchased all those other books would want them, because they were useful for reference, but the original books would still be necessary if you wanted access to feats, classes, etc.)