pparently The Flash show runners decided they didn't need Kid Flash aka Wally West on the show anymore because they wrote the redundant speedster out of the weekly rotation.
Wally did not add much to the show, mainly because his superpowers were just lesser versions of the regular Flash's power.
The few subplots that were written around Wally were often lackluster and forgettable. So using a recent breakup as a catalyst, Wally announced he was leaving Central City to focus on himself.
There wasn't much fanfare surrounding the announcement but Barry did say, "Every hero has their own journey. If yours is taking you on a new path, you owe it to yourself to go."
The show also had to get rid of Wally to make room for it's newest cast member. At the end of the episode, Cecile revealed that she is pregnant with Joe's baby! Maybe the baby will be another speedster? Let's hope not.