Not every anime movie or OVA gets dubbed. Why is that?
The same reason that not every live-action movie gets dubbed into other languages: it's not always profitable.
Dubbing is an expensive, time-consuming process. It's not just a matter of hiring voice actors and getting them to record the lines; you have to adjust the animation so that the mouth movements sync up to the re-recorded dialogue, and in some cases you have to localise the dialogue because it doesn't translate well into English.
On top of this, anime dubs have something of a shaky reputation, with many anime fans deeming them to be inferior to the original Japanese. As such, the market for dubbed anime is considerably smaller than the market for anime in general. Unless an anime (and/or its source material) is already popular enough, a dub simply isn't worth it.