In addition to why they're capable, I'd also like to know why they started in the first place, since it's been shown they can still use phosphorous normally and the arsenic isn't necessarily necessary.
Hmm, "why" is a very, very difficult question to answer, especially in science. We can speculate, sure, but it's only speculation. It's not even certain, yet, if they utilize arsenic in this fashion in the wild. For all anyone knows, this could have been an artificial selection, though more likely, the bacteria have the right genes already, (which is simple to test, once you've determined which genes are responsible; just have to use PCR on an environmental sample to see if those genes amplify).

Now, if they do have those genes, then there's likely a selection acting upon them. Determining that selection is a much more complicated task, however. If that could be determined, then we'd be a lot closer to answering the why of things. I'm not certain as to how to go about determining such a thing, however, as I'm not an Evolutionary Biologist.