Poor Wally West. It's been two seasons since Keiynan Lonsdale made his debut as Kid Flash, and since then... he hasn't actually had much to do. Time and time again, Wally has been pushed aside - so much so that he's temporarily left The Flash, although he'll soon be joining Legends of Tomorrow. But it's baffling that the switch has had to happen at all.
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Looking back on Wally's appearances over the last two years of The Flash, up until his recent decision to leave Central City behind, there have only been two major beats for the character. One was his attempt to gain his speed powers (after Barry accidentally gave them to him and then deliberately took them away), and his relationship with the reality-hopping Jesse Quick. Both of those arcs started with some semblance of promise, only to putter out - Jesse even dumped the poor guy via hologram from another reality, in front of the rest of the team, to add unnecessary salt to the wound.
All this poor treatment had led to Wally briefly ditching Central City for a new life on his own (and offscreen) in Blue Valley early in season four, but he's recently returned to Central City only to... well, not do anything much, despite the fact that Regular Flash Barry Allen is currently in prison. But that's par for the course, because from the get-go, The Flash's writers have had little idea of what to do with Wally as a character.
In the rare times Wally's had a role in Flash's story, he's ceaselessly been dunked on. As a speedster he either gets quickly disposed of to highlight what a threat the current villain is, or to show how unprepared Wally himself is to take on a career as a superhero. That ties into what the show spends the rest of Wally's time doing: Having every other character around him, whether it's his "mentor" Barry (who is constantly allowed to screw up, pick himself up, and try and try again with the support of his friends and family) or Joe and Iris, tell him to stay out of the way because he isn't ready for the spotlight. That, or they flat-out ignore him - something he actually brought up when he decided to leave the team.
When Wally has been allowed to step up as Kid Flash, he's immediately been kicked back down again. Amazingly, in the last two season premieres, The Flash has presented two different scenarios where Barry wasn't around to be Central City's speedster, leaving Wally to step up to the plate. In season three's opening - in which Barry accidentally created an alternate timeline where Wally was the "main" Flash - Barry's arrival on the scene lead to an instant demotion of Wally back to Kid Flash, and then minutes later, him being critically wounded by the big bad and taken out of commission altogether. (It's all right, though, Barry proceeds to wipe that timeline out of existence like it's no big deal, so Wally wasn't technically ever allowed to play in the big leagues in the first place.)
In season four's opening, set after Barry trapped himself in the Speed Force seemingly forever for... err, reasons, Wally at least got to be the hero for a bit longer, but only long enough to get beaten up by the villain of the week so that everyone else decided they'd best get Barry back. So Barry returned, and every other character on the show dropped everything they were doing to focus on helping him get back on his feet and back in action as the Flash, Wally be damned. Either one of these scenarios would be insulting enough to Wally as a character, but to do it in two season premieres in a row? It's almost comically harsh.
The Flash as a show is weirdly aggro to Wally's entire presence, which makes the rapid decision to zap him with speedster powers and turn him into Kid Flash even more bizarre. There's clearly room for more than one superhero on the show, because the series has managed to cannibalise a good chunk of its supporting cast into superpowered vigilantes - everyone from Ronnie Raymond to Cisco and Caitlin, and most recently Ralph Dibny's addition to the team as the Elongated Man. There should even be room on the show for more than one speedster. The Flash, almost to a fault, spent its first three seasons squaring Barry off against a rival superspeed user as the primary villain of each season, and never had a problem with having to deal with that sort of relationship. Why not put a spin on that by instead showing a positive bond between two characters linked by the speed force, like Barry and Wally are?
Weirdly enough, the show is currently doing a great job with a mentor/mentee relationship... but it's between Barry and the aforementioned Ralph, not Barry and Wally. In just a few short episodes since his introduction in season four, Ralph has already had more positive encouragement on his journey to becoming a hero from Barry than Wally has had over two seasons. The show is clearly capable of not just putting Barry in this mentor role, but doing so in an interesting and engaging manner. But why do it for an entirely new character, instead of making Barry the mentor he always should have been to Wally?
There's a perfect environment in Central City for Wally to be cultivated as an interesting character and an important part of the team, but The Flash has spent two seasons actively choosing to not take that opportunity. So it's probably for the best that Wally is hopping over to Legends of Tomorrow to spend time with other people who've also been considered "rejects", before they learned to be heroes in their own way on the Waverider. But Wally shouldn't really have to hop over to another show entirely to actually get some screen time and, more importantly, some respect.