At least that's the way I see it.
Series like Pokémon and Fire Emblem are series that have a constant shift in cast members, so it only makes sense to have their new characters get in. Although, for FE, I think Marth and Ike should stay as they are very iconic to the series, moreso than most other lords.
For Pokémon, you pretty much have to do it that way, as there are now over 700, so with the Pokémon they deem their most marketable, those Pokémon have the best chance.
The problem with Roy and Mewtwo, is that Mewtwo is very very relevant to X & Y, whereas Roy isn't relevant to what is happening in Fire Emblem besides his DLC appearance. Marth and Ike both have 2 games devoted to them, so they have more series importance to them than several other lords including Roy.
The difference is that Mewtwo is still relevant to his series, while Roy pretty much isn't even if he did appear in Melee. That hardly matters now really.
Other series however, have classic and iconic characters who aren't recent, and don't really have a changing cast. DK is one of them. The main cast hardly changes, besides the forgettable Tiki's and new baddies in Tropical Freeze. K.Rool will always be THE Bad Guy of DK, as Ridley will be to Metroid. The casts are subject to heavily changing. That's why recent events hardly have an effect on them.
The only way K.Rool can lose out to a character with a more recent appearance, is if Dixie or Cranky gets in over him, and that may not even be because of recency, because the other two are very iconic as well.
EDIT: @
Morbi
, sure, F-Zero has relevance to Smash Bros., but besides that it's lacking compared to several other series. That is fact.