After Clinton won New Hampshire in 1992, every channel’s evening news and every non-right-leaning newspaper (meaning almost every newspaper) promoted the narrative that Clinton’s second-place finish was a big deal. The media telling voters that the candidate has done something unexpected that will give him momentum gets the voters to give the candidate a second look, to view him more favorably (he’s winning!), which drives up polls, which gives you another cycle of momentum, and so on.
The media-driven narrative of momentum used to be able to create actual momentum. But that only works when you have a unified media narrative to get the snowball effect started. And a unified media narrative is precisely what America no longer has.
Rubio did nothing to warrant winning a “comeback kid” designation in 2016. But imagine if he had, and then had been christened “the comeback kid” by CNN, and even maybe by Fox News. He still wouldn’t be called that by Rush Limbaugh, and certainly not by Breitbart (in the tank for Trump), or The Blaze (in the tank for Cruz), or MSNBC (in the tank against whichever Republican looks most electable).