More possibly, time constraints. The VA are busy with doing the anime roles and they have to du then by one episode each week with a movie per year....You mean his Japanese anime voice?
OLM was involved with production too. The Origin was a collaboration between OLM, Production I.G., and Xebec. The latter two have helped with the regular anime before (though not as much as here). Not to mention, all official Pokemon-related products are internationally owned/licensed by TPCi, so it makes no sense for them to hand it to another company due to "lack of rights" when they own it. If this wasn't done in-house, they probably have another reason for leaving it to another company to handle the dubbing.It's probably because a different studio made this special so they don't have the same rights.... as they do over the regular anime, I am assuming.
Also, I don't get why it matters if Red is voiced by a female or a male, especially since the former sex (more often than not) is viewed to suit the role of a young boy better due to common general pitch... unless it actually is a young boy doing the voice (Aaron Dismuke, voice of Alphonse in the English dub of Fullmetal Alchemist, when he was 12).
They sub-contracted this because maybe it's easier to them su-contracting an entire group than bringing an extra pay to the usual VA. And because this is non-profitable for TPCi and has no effect on the main anime, they decided to do with this route.