|This thing is more highly evolved than me.|
The problem with my reasoning here is that when I think "infrared vision" I think of the night-stages in the Modern Warfare games, where my lovely IR goggles turn the dudes I need to shoot in the face into bright orange blobs against a dark background. While this is a more or less realistic portrayal of how modern infrared imaging gear works, it ignores the fact that IR-imaging gear is specifically designed and tuned to detect a very narrow range of infrared radiation; in the Call of Duty case that would be whatever wavelength corresponds to the ~37C body temperature of the guy trying to kill me. We've gotten extremely good at building wavelength-specific sensors as long as we know what wavelength we want to look for, so something like this isn't all that difficult from an electronics standpoint.
As I mentioned in an earlier post though, monochromatic sensors are pretty much nonexistent in nature, for various evolutionary reasons I'll discuss in a minute. That's problematic, because in the grand scheme of things there's a ton of infrared radiation around and a lot of is close to the same temperature/wavelength. Basically anything warm is radiating at some infrared frequency, including water, the ground, and the air. If you're looking for plants or cold-blooded animals to eat and/or not get eaten by, an IR sense is going to be entirely useless, since they're the same temperature/wavelength as their surroundings. You might have slightly better luck with endotherms, but they're still not THAT much hotter than their surroundings, and (contra the Discovery Institute) our designer apparently wasn't intelligent enough to give us infrared eyes with frequency-specific lock-in amplifiers on them. Essentially, unless you have extremely good spectral resolution, seeing in infrared would just look like the equivalent of a whiteout most of the time. Since the evolutionary advantages of seeing a constant whiteout are dubious, it's not too surprising that no random mutations with an infrared sense stuck around long enough to evolve the resolution they'd need to go all Splinter Cell on woodland creatures.
Practically speaking, visible light is a lot more useful. Assuming you're out in the woods looking for food, there's really only one source of visible-range light (the sun) to deal with; everything else is just reflecting varying amounts of that source (this is another advantage, as IR light tends to be absorbed by things rather than reflected). At night, if there's any kind of moon, you'll get the same spectrum from the reflected sunlight. So if you've only got a broadband detector (hello eye!) it makes a lot more sense, if you need good night vision, to evolve in the direction of making your visible-light detector work better (see cat eyes) than developing an IR sense that's going to be evolutionarily useless unless it's tuned to the precision of a military-spec lock-in amplifier. The aforementioned IR sense that some snakes have is more a backup than anything else; it's low-resolution and mostly just helps them figure out the general direction of prey until their senses of visible-light sight and smell can do the fine-location work. It's one of those mildly-useful-but-they-could-probably-live-without-it quirks of natural selection.
So as usual, video games have made me stupider. Possible next entry: why doesn't jumping on turtles give me extra lives?
Thanks to quora.com for the majority of the information here. No thanks to useless Wikipedia this time.