Short answer: Bulk Up isn't necessary in-game, as you should be consistently attacking to pummel your foes down. Boosting moves are almost never necessary, especially in Sword and Shield.
Long answer: It's a waste of a move in-game. Most battles are very easy and over in a few turns, you don't need to waste time with Bulk Up when you could just be attacking. It also leaves you open to getting KOed. In some games, boosting moves are nice to have, but that isn't the case in Sw/Sh. Every battle except perhaps Leon is quite easy, and even Leon can be dealt with fairly easily due to the abundance of EXP Candies in the game, or good old brute force.
Corviknight also has a few (albeit not very many) other moves in its repertoire it can use to greater effect than Bulk Up in-game. Why not use Body Slam, easily accessible neutral coverage with an added bonus of paralysis? Or perhaps U-turn, for allowing Corviknight to switch out of an unfavourable matchup, doing damage all the while. While Corviknight doesn't have the greatest Attack stat, it's enough to get the job done. If it wasn't, why use Corviknight in the first place?
Hope I helped!
