I'll give you the second part; maybe Wii U specifically (3DS was successful, just more like PSP levels success and not DS levels success) being a dud is what convinced LEGO to not do exclusives anymore. But then that's contradicted by the fact that, if Nintendo paid for exclusivity, it shouldn't matter to LEGO if those exclusives didn't do well? They already got their money right?
Oh and there's now this:
What is this decision? Why does this game not have cross-play? It's not even a competitive game. Local 2-player co-op is cool (and expected), but no cross-play is weird.
Seems like no cross-play even between PS5 and PC; none with Switch could at least be explained by Nintendo being Nintendo.