Most articles speaking about Nintendo's approach don't point out to why. We have one where Furukawa talked about bringing new crowds through Smash (which basically explains all the third-party characters), but that certainly doesn't apply to Byleth, there's that one where an employee says everyone is a must buy, which doesn't really mean anything; same for Reggie's interview. I don't expect Nintendo to reveal why they went with the characters they did, but what little there is to speculate on doesn't really suggest anything better than an attempt at advertising.
Speaking as someone who used this very same argument months ago (and kinda regrets it in hindsight), I came to accept that even if there are underlying patterns over the fighters' selection that doesn't mean that they are going to apply to every single character.
What I mean by this is that, while I still believe that "drawing new crowds" was one the major objective behind FP1, Nintendo probably thought that 2-3 fighters of that kind were well enough for their plan to come to fruition.
Banjo and Byleth being chosen among Joker, Hero and Terry doesn't make their M.O. incoherent, it just means that this "rule" was a lot more lenient than what we initially thought.
I think a more clear example of what am I saying is how some people think that the inclusion of Incineroar and Plant "ruined" Ultimate's base roster because it signified that the developers didn't focus entirely on long awaited fan-favourites to make the most out of the few resources thay had dedicated to newcomers (though of course that was entirely on the community for having that kind of expectations).
The bottom line is that, at the end of the day, characters get chosen mostly for their individual merits (whatever they may be or how we feel about their value), and those merits can help them overcome any "patterns",genuine or not, so those should be treated more as a general guideline than as ironclad commandments.
Like, even if we start to notice some patterns when 3+ characters get revealed for F2P, that doesn't mean that those patterns will apply to every single character, but even then it also doesn't mean that those patterns weren't valid.