Anthropologist of the South Pacific foodways and agriculture here.

I can't fully answer the question because I don't think anyone can truly objectively explain why pop-culture and discourses have largely excluded Polynesian-American contact, even if it is worthwhile pointing out patterns that could partly explain this lack of popularity.

In my anthropological sector and people do and have been talking and arguing in favour or against Polynesian-American contact for a long time (decades), but perhaps the biggest factor explaining why popular discourse has ignored it is the fact Polynesians and Indigenous Americans interacting has only been decisively concluded a couple years ago, with human DNA demonstrating genetic admixture circa 1200 shared between Pa’umotuans, Marquesians, and Mangarevans (island groups were the study was conducted, does not mean other Polynesian peoples are not related, also very important to note the three are rather far apart and distinct cultural groups even if they did sustain contact and trade with each other) and South American DNA closest to modern day Colombians (https://www.nature.com/articles/s41586-020-2487-2).

Indeed, before 2020 there was no veritably unequivocal evidence for contact.

Sure, linguistic evidence for sweet potato and the fact it was in Polynesian islands pre-European contact made a strong case, but DNA studies for Polynesian sweet potatoes support the theory the root crop floated through the Pacific and arrived in the South Pacific tens of thousands or years before the Austronesian expansion even started (https://www.sciencedirect.com/science/article/pii/S096098221830321X), and those who argued chickens were introduced to South America by Polynesians are contradicted by further DNA evidence showing South Pacific chickens to be wholly different from South American ones (https://www.pnas.org/doi/10.1073/pnas.1320412111).

It's worth noting the human DNA discovery made huge noises in academic circles interested in the region, and perhaps even more important to remark the study does NOT prove Polynesians reached the Americas any more than South Americans reached Polynesian islands, with some arguing the latter is more likely based on the evidence we currently have.

Lastly, there is the problem of conspiracy theorists frequently presenting South Pacific islands and societies as remnants or successors of ancient advanced civilisation (Lemuria for instance) that has long fallen for their apogee. I mention this particularly because this mythical ancient past often includes contact with other (or the same) ancient advanced civilisations in the Americans (North, Central, and South), with pseudoscientific arguments swearing structures like Nan Madol are linked to Mesoamerican pyramid structures and were built many thousands of years ago before collapse and are proof Lemurian civilisations existed. It is likely the fact lost continent/Lemurian conspiracy theories argue through pseudoscientific proofs that the South Pacific and Americas were linked as part of an advanced civilisation ultimately destroyed in time immemorial as a mainstay of their argument makes it a touchy subject in popular media to discuss inconclusive theories in scientific circle that can easily be interpreted as proof for mega-advanced ancient spiritual mastermind builder civilisations or something of the like.

How the hell is nobody talking about this?? It’s bad enough that we still talk about Columbus as much as we do, and only fairly recently does it feel like American classrooms have added Lief Erikson as an asterisk next his name. What’s been keeping the Polynesians out of the narrative?

I wanted to address this question even though it might not be the main thrust of your post I do think there are some important points to be made. First it is not bad that we talk about Columbus not bad at all. It's bad to talk about him only as a heroic explorer and to ignore his actions on the island of Hispaniola and the terrible consequences of them, but Columbus voyage led to an almost immediate start of European colonization with Columbus himself as a prime agent. This rapidly led to a huge change in the culture and social order of two continents.

Columbus' voyage is more important because a direct consequence of that is life irrevocably changing on two continents and having profound effects on the entire world. Lief Erikison by contrast is an asterisk ultimately his voyage had few lasting impacts beyond some old stories and wood for Greenland. The same for this Polynesian contact is interesting to note but the exchange of the sweet potato and a few people pales in comparison to what happened after 1492. To understand the history of the Americas after that date you have to understand European colonization which invariably involves taking about Columbus.

There is a pop history idea that it's just who is first who matters, but for how much these things get discussed it's a lot more about impact. These Polynesian South American contacts will certainly be essential for discussing the spread of domesticated crops but ultimately for the larger historical narrative they are always going to be an asterisk.