As I said before, Decidueye and Incineroar would be the only two picked between the Starter set because their concepts would interest players (much like how Greninja being a ninja-based fighter did) and their designs can easily be adapted into Smash. Primarina has the issue of being of a rather unfeasible design to work in Smash's setting. As for the concept....an archer and a wrestler are going to be
much more enticing of fighting concepts than an idol/siren. If neither the archer or the wrestler are picked, the other options outside the starter set are much more believable.
If Primarina had a more workable design and interesting concept, I'd be seeing it as a legitimate possibility. I have no "problem" with Primarina specifically (even if I like Brionne more because it's basically Astolfo as a Pokémon; don't tell me you can't see it too :V). Just that there are too many inherent issues with it that make predicting it over all other options unreasonable.
As for what Incineroar has over the other options, I'm going to try to keep it as brief as possible.
When the initial proposal for Ultimate's design (which is usually when the characters are decided upon), this was
before Sun and Moon were even announced to the public.
So Sakurai would have to do what he did with Greninja and look at the various concept arts to see which Pokémon would "fit" the most in his head.
Just of the Starters alone, Incineroar shows the most potential from the concept art
by itself. Other Pokémon could have interesting enough concepts that he could have seen earlier than us, but there's a
lot Sakurai could think of with Incineroar's concept of a heel wrestler tiger, especially with Tiger Mask being as big as he is in Japan to the point he's been defictionalized and portrayed by several different people in Japanese wrestling history.
And bear in mind who
else is working on Smash Ultimate. Namco-Bandai. Surely
they could help inspire a brutal wrestler style not yet seen in Smash with cues from one or two of their
own characters?
The concept art also implies something rather interesting. Incineroar would be present in anime among the main cast. Particularly with
Ash.
And so far, Ash has a Torracat that has a fierce yet friendly rivalry with another Incineroar, with strong implications that it will be resolved with an Incineroar vs. Incineroar match.
So what does it mean for back
then? That Incineroar was pretty much said to have future prominence, something Sakurai considers.
Also of note that the following year, for the
20th anniversary movie no less, Incineroar was featured as the antagonist's main Pokémon, and one of only three Gen 7 Pokémon to be featured in the entire film (outside of brief cameos in the credits). So it shows that Incineroar is being pushed considerably.
None of the remaining options have this sort of claim.
Does this mean Incineroar is a lock? No. But it has the most going for it among the options.