Judging by Sakurai's facial expression, words, and the rumors of trying to get Minecraft since 2017. I wouldn't be surprised if Steve was the hardest character to make for any Smash game period because of his building mechanic due to all of the changes they had to make to the code, engine and stages to make it work.
I'm assuming this is basically the history of Steve being in Smash:
2017: Nintendo begins talking with Microsoft about Minecraft. Sakurai believes he doesn't have time to program Steve the way he thinks Steve should be depicted as a fighter on top of all the veterans and an already small number of unique newcomers, so they put it off for later.
2018: In the meantime, they talk with MS at E3 2018 about Banjo & Kazooie, believing them to be an easier character to program while also satisfying Nintendo fans, and ensures Microsoft remains on board with Smash.
2019: Banjo gets in, and there is a suspicious lack of Microsoft costumes in their Mii costume wave. He and Hero make the first pass sell enough to warrant greenlighting a second pass, Nintendo takes this opportunity to talk with Sakurai once again about Steve, and once the division that worked on Terry was done with him, they got to work on reprogramming the stages for Steve's blocks. Obviously for such a character with limited animation, this is the hardest part, as opposed to Min Min with her ARMS gimmicks.
2020: Steve gets announced for Smash as the second fighter in FP2.