Soul Edge was good, Soul Calibur 1 even better...
But Soul Calibur II added some really fun characters and hit the mainstream appeal required to host tournaments.
Soul Calibur III was a dud, simply didn't improve the game in any way, imo. And nobody played it.
Soul Calibur IV wasn't that much better, although I personally liked the more tactical pace. But it came out in a time that there were very few quality fighting games (right before SFIV), and hence, there was some competition. Until Hilda became broken...
Soul Calibur V was a huge flop, probably because half the characters got replaced.
Soul Calibur VI grew its player base, maybe because of Covid, or just because it brought a lot of characters back.