My guess is that it has something to do with the terms of their licence for the dub. Maybe they would have had to pay more to include it in more than one SKU. I'm reminded of the eyebrow-raising situation with the ADV dub of Eva being on a separate set of discs and only in certain (massively overpriced) SKUs. That's only a guess, of course. We could really do with more transparency from AL about why these things happen.
It's frustrating, since I'm only interested in the OAV version. I would have paid collector's edition pricing in a heartbeat, but couldn't justify £150 for four episodes, a chopped down clipshow movie, and a book. No doubt they would point to the 200-page artbook as the big expense on their end, but if Udon can release a 300-page translated artbook for Bubblegum Crisis at $60 then I have to wonder.