As one of the people who firmly believes only one will get in and not both, it's got nothing to do with them as characters.
DLC is limited, and I don't think they're going to use two of the remaining spots on DK characters. If there was a large amount left, sure, but I'm only expecting around 2-5, and just don't see more than one being from DK.
EDIT:
Ohhhh, in that case, yeah, there was really no reason why people would guess one or the other. They both could've made it on the base game for sure.
That sort of thing (Only one newcomer per series) happened with a lot of franchises, and always has. I like to compare the K. Rool vs Dixie thing to Wolf vs Krystal from Brawl's roster speculation days. Mario is another series that got hit hard by this belief; Mario newcomer speculation sorta died after Rosalina's confirmation. Captain Toad was pretty much the only name for Mario that came up after she was confirmed. Bowser Jr was a frontrunner from pre-Brawl all the way up to Rosalina's trailer, at which point he pretty much vanished.
Go back and look at pretty much any roster prediction list, and you're likely to find only one newcomer per series, except for, like, Pokemon.