May was one of Ash's best traveling companions. She was funny, energetic, and amicable. Additionally, not only did she catch all sorts of Pokémon, but she used them to compete in various Pokémon Contests and put on spectacular performances. Longtime fans of the show love her and wish she'd show up more often like Ash's other old travel companions. However, she only reappeared for a few episodes of Diamond and Pearl and has since only shown up occasionally in brief, voiceless cameos. Unfortunately, while it would be nice to see more of May in the modern anime, her absence is not as simple as one would think.
The real reason May can't come back is that her voice actress, Kaori, is virtually incapable of performing. She's been suffering from a long-term medical condition that prevents her from using her voice properly. For a better idea of what's going on, here's what happened to Kaori and why she may never return to Pokémon.
Kaori started voicing May in 2002 and voiced her throughout the Advanced Generation series. Naturally, she left the show in 2006 when May was replaced by Dawn for Diamond and Pearl. At the very least, this form of leaving the show was to be expected.
Kaori started feeling something was off about her voice in 2007. She wasn't sure what it was, but she felt it wasn't a major impediment at the time. She was even able to reprise her role as May for Episodes 75 through 79 of Diamond and Pearl. This, however, would be the last time viewers would hear May speak.
In 2012, Kaori revealed in a blog post that she'd been diagnosed with spasmodic dysphonia. This rare variation of focal dystonia severely impedes speech and makes it difficult to understand what the afflicted person is saying. Having this disorder makes it difficult to perform tasks that involve using one's voice.
Not only did Kaori have to step away from voice acting, but she also had to stop singing, too. Her band, Spunky Strider, made the heartbreaking announcement in 2014 that they'd be breaking up. Kaori isn't directly responsible for this, though it's possible that her condition was a factor.
Since then, Kaori has been keeping people updated on her condition through social media. She's been trying all sorts of treatments and medicines, both Western and Eastern, but she's yet to find any lasting solutions. She continues to hold out hope that an effective treatment will be found.
As for whether May's voice will be replaced, that's unlikely to happen. Japan holds voice actors and actresses in too high a regard to simply swap them out when it's convenient. Voices only get replaced when the actor dies, goes on maternity leave, becomes too expensive to hire, or gets involved in a scandal. Even then, the characters they voice will likely be assigned minor, nonspeaking roles for a while. If May gets the chance to speak again, it probably won't be any time soon.
For the time being, May will continue to be relegated to minor cameos. Her voice actress is too sick to reprise her role, but she's also too highly regarded to be replaced. Although many fans might want May back, it's important to understand Kaori's situation and wish her well.
More information about spasmodic dysphonia can be found on the National Spasmodic Dysphonia Association's website at www.dysphonia.org.