Because ultimates and their relics and titles are evergreen content. More people have the field commander's coat from the seal rock achievement than they do top 100 ranked, because its evergreen content, its within someones reach.
While ranked feast seasonal stuff was cool looking and as a walking advertisement to try and incentivize ranked play, it didn't pull in the numbers they wanted. Which is why every new mount and new armor set that might've been for feast originally, gets released as part of the season pass style of content, where any player can get it, that it just asks time and patience to do so. In a sense, ranked pvp for top 100 players was a mistake that creates a walled garden of a community, which is not what they want. Those walking advertisements that can come from any style of content like a mount, weapon, armor, title, or even minion are to encourage old and new players to go play and earn that same thing years later and therefore naturally get players to interact with that very content.
Like when they have the FF16 crossover event, that is to get people to engage not only with the questline and solo encounter, but also to get people to interact with the gold saucer in a more natural form. And just recently with Yokai Watch, that incentivizes even more for old and returning players to engage with the event naturally by also introducing a new framer's kit that just simply requires interacting with the gold saucer's content. Same with the Garo event, but for pvp.
Because while its nice to get a tell or two about having a fancy and rarely seen mount, that same mount also kills any interest in the very thing you want more people to join in on. Because the inevitability will always be:
Random Player: "Hey where did you get that cool mount?"
Top 100 Player: "Oh I got it from pvp."
Random Player: "Cool, I will work on that later."
Top 100 Player:"Sorry this was the prior season's reward, its not available anymore.""
While true this can still apply to seasonal pass stuff, there is at least the caveat, that other players can gain interest within those same months as you did to earn that very same thing. And that previous seasonal rewards from the season pass, at least according to Yoshi P, will be available later.
Which is why they can only give ranked players a framer's kit. Because that item is the least likely thing to cause disinterest in the content.