I'm in despair, the glaring flaws in the game industry have left me in despair!
Seriously though, one of the bigger problems they're going to be having with sales is, there are just too many FPSs on the market, especially modern warfare shooters. That we even throw around terms like "CoD clone" is symptomatic of the problem. Even if a game were a radically innovative leap forward in the genre, I imagine a lot of us still wouldn't be interested, just because we're weary of the very paradigm of a cover-based FPS. I, for example, wasn't planning on buying Homefront even if it was hailed by fans and critics alike as the best thing since hot chocolate.
Still, I agree that 73 isn't actually bad, and most of the reviews I read had very favorable things to say about a lot of the game's aspects. This is one of the reasons I don't like scores, because nobody has the same metric. I could go on a big spiel about review theory, but I don't think anybody's interested in that... Anyway, the point is, most of the outside world seems to think that 73 is mediocre at best, so whether or not 73 really is a pretty good score, it'll likely hurt sales.
Of course, it hasn't actually failed yet. It could end up selling very well, despite the scores. I think Medal of Honor made a profit, after all.
Whether it's deserved or not (I'm not willing to make that judgement, having not played the game), it's kind of tragic how Homefront seems to be headed towards a tragic ending.