From what JMS has said it seems that he and Warner Bros. just didn't think it "fit" with the rest of the movies. The other films were all produced during the period of the series production and focused on the original story and characters. Even A Call to Arms was as much the last Babylon 5 adventure as the set-up for Crusade. The Rangers pilot was a different animal. Also since it seems that the B5 feature film could end up overlapping in time with the period that would have been covered in a Rangers series it may be that the studio wants to hold the TV movie back to coordinate its DVD release with that of the feature film.
Regards,
Joe
