With The Flash writers saying the true reason Wally was written off was due to having too many speedsters to work with, putting Wally on the Waverider is a way to keep the character in front of fans in case they want to bring them back later, while still allowing The Flash to tell its current one-speedster story without constantly giving updates on Wally.
Wally has been largely written out of the Season 4 story of The Flash and is on a journey of self-discovery, and where better to do that than on Legends of Tomorrow?
As Marc Guggenheim mentioned to Comicbook, there’s more than just one slot to fill on the ship.
While Constantine is guaranteed a seat on the Waverider for at least a couple of episodes, there’s still an empty chair that could very well be filled by The Flash’s Wally West.