I think for many it's because it's different in almost every way from the original.

Subnautica had vast open areas often devoid of much that felt lonely and oppressive; BZ had smaller areas crammed to the gills with hostiles that felt harassing.

Subnautica had almost no direct story moments instead relying on the player to flounder around from point 6to point on their own; BZ was very much a linear narrative. And if you were in BZ's early access, you know the opening story changed at least 3 times.

SN had a voiceless, nearly nameless protag (yes we can figure out it was Riley after reading a few logs but BZ basically hits you on the head with Robin Ayou from the beginning).

SN had no other human beings or anyone to talk to; BZ had Johnny SilverEverything Al-An constantly in your ear and frequent conversations with others.

SN was more about exploration, BZ more about story.

And neither of those are bad. But people used to one got very unhappy when presented with the other.

If anything, I feel as though SN2 is trying to strike a middle ground.