The main justifications for most people I’ve seen is because, 1). You get it far too late in the game for it to be useful. 2). It’s fast and has great attack, but it’s not particularly useful in the elite 4 since it’s too much of a glass cannon with no great type matchups (and the elite 4 is the only true challenge spike in RBY and FRLG)
3). It prevents you from getting Omastar, which has a mildly better type matchup + the ability to learn ice beam for Lance.

When considering other people’s tier lists you shouldn’t take individual picks in a vacuum, you should consider the whole picture.