Because it is either an asshat move or a move that only results in lost revenue by throwing good money after bad. Let's tackle the first first.
"So they are still letting people buy stuff for a game that is already having it's remake coming out? They are charging us for things that are entirely pointless because it isn't going to matter in regards to Nova. This is nothing more than a greedy moneygrab." Now, maybe that isn't the case. Maybe they just want people to be able to play the game and have the store open so they can pay for the server to stay up. But the perception from everyone who isn't playing the game is going to be "that is a moneygrab and they should be ashamed of themselves."
Now for the bad business part:
How many people were having to spend money for DUST to be profitable? Now how many of those people would still spend their money in order to purchase Aurum in a game that has been announced dead not once, not twice, but thrice now? (Rouge Wedding, "5/30/2016 is the last day", and with Nova being the spiritual successor) Hell, the fact is that most people were angry with the Warbarge system because "wait, isn't this game dead and you are including ways for us to spend lots of money?" shows that people would be unwilling to spend cash now that the game is officially announced dead. I think it would be a lose of money for sure.
The other is also perception. Let's be 100% honest, DUST was not and is not a good game. I would give it 2/5 for some cool ideas and implementations but lag, performance issues, and mediocre play drag it down. Sure, DUST could be your favorite game as a 2/5, there is nothing wrong with that; I enjoyed DUST quite a bit even with knowing it wasn't that great of a game just because I enjoyed it. But Nova is supposed to be the new New Eden FPS. CCP absolutely does not want DUST as an albatross around the neck of their new baby. The last thing you would want is for people to be able to look at DUST before playing Nova if you were hoping to get a fresh impression.
http://eve-offline.net/?server=dust gives some numbers for the trend of our game.
Past year average: 1,735
Past 6 months: 1,472
Past 3 months: 1,129
Past month: 970
Past week: 760
Those numbers would only drop lower and lower if the game was allowed to exist as a zombie-esque, neither dead nor alive.