Probably because, for someone new to Chicken Invaders, some of them might not make much sense without the context of the original episodes - for example, why would U.C.O. No.4 be in meteor storm when the other three aren’t?
I am not at all against the idea, but I think that there should in that case be at least a small pool of potential bosses. Certainly only having one boss per category at the moment gets repetitive when you play a few so I’d like to see some diversity - but it shouldn’t feel arbitrary to new players, either.
That said, I agree that Shoot the Core should definitely be a supernova boss rather than a standard wave boss - it’s kind of a weird one to make a “normal” boss fight, except of course in a boss rush, where anything goes.
Also, along the lines of adding Feather Fields - I don’t hear the “Popcorn” wave get mentioned too often (probably because it’s kind of a weird one - giant corn cobs) but for the sake of continuity and completeness I’d also like to see those pop up (pun not intended) around stars and “inferno” planets (in addition to the feather fields getting implemented, of course. The original game mentioned “Yolkon 7” [I think] - imagine if the feather missions took place on giant, egg-shaped chicken planets on the open map! Or just random clusters of floating feathers because why not.).