#1 To actually make all of the reigns meaningful
The worst thing that you can do as a booker is to hot-shot a title or have it switch from one person to another, without giving a WWE Superstar the rub that comes from the Championship reign. Let's assume for a minute that Humberto Carillo had managed to defeat Andrade to become the new United States Champion.
It would have completely undermined and overshadowed Andrade's reign, who became the United States Champion not long ago when he defeated Rey Mysterio at a WWE Live Event at MSG. You could say the same thing for The Fiend as well, whose reign would have lost much of its sheen because the story right now is that he simply cannot be beaten by anyone at all, however hard they try!
Having these reigns stretch all the way to WrestleMania would help in building formidable Champions and the audience would actually look forward to a title change. 'Who can defeat The Fiend clean?' is a question that the WWE Universe will want an answer to, and WWE can provide the answer at WrestleMania 36.
For a current Champion to drop the title at the Royal Rumble would have been a total waste!