I am not sure if the devs will read this...
I think it's ironically because the Yooka-Laylee cast and setting has more potential. Yooka, for starters, is from the word go a deeper protagonist than his N64 analogue ; he's not solely defined by being the 'nice guy' of the duo, being more defined by a thirst for adventure and an actually in-universe unhealthy addiction to adventuring (to the point that he will jump into any opportunity for a quest by NPCs, without getting all the details).
Meanwhile, Laylee is also less defined by being the 'snarky one', with hints regarding the fact that she doesn't want to go on on adventures as much, is actually less of a jerk than she want people to think (when faced with quests that truly go wrong, Laylee become genuinely uneasy and shift the blame to Yooka).
There are also hints about some serious differences in their relationship, especially with some of the jokes and dialogue points in the second half of the game.
On the villain side, Capital B is actually surprisingly interesting and V.I.L.E. is a much larger scale organisation involving a massive conspiracy and a long-reaching evil, with a much more grandiose goal than the petty machinations of Gruntilda. Likewise, the loyalty of V.I.L.E. officers is not as clear cut as the villain dynamic of the Winkybunion sisters (won't give spoilers on that point).
Add to this a setting and theme that revolve specifically about writing, stories, the rewriting and creation of reality through magical writing, and the struggle of various factions over the control of reality and we have the recipe for a story that can go way, waaaay beyond the original Banjo-Kazooie model in terms of humor, drama, story arcs, and depth.
... which lead to the game's paradoxal problem: Yooka-Laylee's cast and setting could be so much more. But in-game, what we got is only Banjo-Kazooie with barely any hints of what it could be.
We have elements for a grand story that practically write itself... and instead, we were given a re-tread of Banjo-Kazooie almost 1:1.
And that is why opinions on the story are so utterly mixed. Because if you read the character profiles and get the hints, there's definitely something here. But it is not properly used. Simply put, we have unused potential.
And that is why I think it cause such problems ; it's one thing to have a simple story when you have a setting that obviously cannot be pushed farther. But the Yooka-Laylee setting can very obviously be pushed farther.
And that's the reason, I feel, why people make such a big deal of the story being simple and bash it. We have the potential to have something that could easily surpass the source material... and instead, we have an accurate throwback.
If you can accept it as what it is (a throwback), the story is fine. But for those who had wished for the setting to shine on its own and grow past its roots, it leave an empty feeling. Like having a cool magician on a stage but never having him pull out anything out of his hat.
That is, at least, the feeling I get from speaking from those who were disappointed and actually explain themselves instead of just saying "It sucks".