Even if there is an answer, part of it has to be about how it is different for each game. You can imagine that there are different development processes that are used for different games. And some more complicated games are just generally harder to convert, than say, a simple indie game.
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Even if it was just moving the code over so that it works on PC that still means you have to test it under a variety of configurations and drivers... and if that was ALL they did, then that's a pretty piss-poor port. You would HOPE they'd incorporate features that would allow the PC version to shine rather than just making sure the code on the 360 version translated to windows well...
If all you want is a bare bones experience, an emulation would be sufficient. Also from what I've seen of Alan wake, that game was practically ad supported (Duracell, Ford/Lincoln/Mercury, Verizon)... I mean, I have the game on steam, I played it a bit and lost interest. I saw it enough to see how many ads were plastered all over it. Maybe they got paid extra for moving their advertisements to PC?