...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).
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.
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.