Because that's how they get you by the balls
Frontload the pass with the cool hype picks like Steve and Sephiroth to get people on board and then drop the less exciting characters like a new Pokemon or Fire Emblem by the time everyone's already bought the pass.
We need to be real here. I get that it's fun to doompost about how Smash DLC cycles have ended with real "disapointing" picks like Corrin and Byleth, but really, they
couldn't have come any earlier in their DLC cycles.
Corrin was decided on
after Fates was released in Japan in June 2015. They were revealed in December 2015 and released in early February 2016; all before the North American launch of Fates later in February (and according to Sakurai, that release timing in-between the JP and international launches of Fates was part of the reason why Corrin was selected). Also keep in mind that Bayonetta, despite being the last character revealed for Smash 4, was in the works earlier than Corrin was; they were the last fighter to enter production for Smash 4 because of the release timing of their game.
Byleth was decided on in November 2018, but according to Sakurai only began production when he gained access to a near-final build of Three Houses a short time before launch. There was no way Byleth could've released any earlier than they did, considering Sakurai wasn't even able to wait for the final release in order to keep up their production schedule.
Sword and Shield has been out for over a year now. If they wanted to make a rep from it happen ASAP they could've released in place of Steve or Sephiroth. That isn't to say that a Gen 8 rep absolutely won't happen because they've waited too long though; like I said I just think the chances of it decrease if they miss out on anniversary synergy.