That makes sense to me. StarDock only acquired the online distribution rights after the initial push out to retail shelves had died down--despite much critical acclaim, that initial push wasn't very big and it's unclear to me just how many hard copies were even manufactured.
It's a little unclear to me what the financial situation for this game is. The Russian developers, Elemental Games, went bankrupt a while back, but obviously Cineware Marquee is the publisher who owns the game, and they're still making some money (on a royalty basis, I assume) selling it through StarDock.
Certainly, the digital download is a good way to push the product out to a lot of different consumers, without the huge fixed costs of creating another batch and the challenges of pushing out a months-old game w/a dubious title but excellent gameplay to the big retailers.
I kinda think that EB Games, Game Stop, Wal Mart, and a few other giants pretty much control the shelfspace given to PC games in North America. Given that they all have pretty similar purchasing strategies, a game like SR2 just might fall through the cracks and not make any money even if the publishers incurred all the costs of a second printing.
I would be curious to know how many online copies of Star Rangers 2 have sold since this game went into online distribution (I bought mine through StarDock based on the good experiences I've had with them in the past, but I've seen it advertised through one or two other online distribution services as well).