No one is arguing that TFATWS!Bucky is a totally different character. But the audience for TFATWS and the MCU movies isn't the same and people who only or mostly like the show might just want to request and offer TFATWS fic specifically.

Like, Picard from Star Trek: Picard and Picard from TNG are also the same character, but the shows are still nominated as stand-alone shows rather than smushing them into a giant umbrella fandom.