According to the rumors I've heard, the reason for this is that when the BBC announced the recovery of Galaxy 4 well in advance of its release on The Aztecs DVD, they didn't sell as many as had been hoped.
Not least because so many people won't double-dip, and why should they? Lots of us who already bought The Aztecs on DVD aren't going to buy it again for an extra extra feature, even if it is a previously missing episode. Maybe if it had completed a story, sales would have picked up, but one episode...? nah.