Because it's fashion, fad, whatever you want to call it. For several years, the string-through bridges caught on, and the tennis ball subject it is gets batted back and forth across the net between 'it's the greatest thing ever' and double-faulted on the other side of the court with 'it's all BS'. But enough people think it's some big ju-ju that's going to give them something that will mystically improve their playing. It's just as well, Kirk Hammett would probably not let Fender print Ouija board graphics on a bunch of basses, which would probably be as effective . . . .
Evidently, Consumer Market Research has told the honchos at Fender the ju-ju has faded, and so the Ultra's . . . aren't string through. Horrors !
OK, I'm ready, let me have it. I don't hear it, can't feel a difference, and as the Almighty has a sense of humor, you're STILL going to have the Floppy B-String Blues.
Me, personally? Nope, doesn't do anything for me except probably buying long scale strings . . . . . that probably aren't long enough. I feel for the heartbroken flats guys that can't use them, Life is Just Unfair.