I have been staring at my computer screen for nearly ten minutes trying argue against this, but I got nothing. I will say that while they share plenty of assets, Bioshock 2 has plenty of it's own unique assets. I'm just not convinced that would have been a significant hurdle for a prequel Bioshock.
I'm just making the point that a prequel would've greatly reduced if not rendered infeasible the ability to repurpose Bio1 assets; Rapture proper would have been in a completely different state to what we'd seen in the original game.
The game still costs 5 euros. How much is it in the US?
$5 for the base game or $10 for the bundle.