I'd like to understand the reasons why the switch was made, because we are evaluating which dvcs to use in our company (also a WISC shop).
It was just a matter of convenience. We have quite a few open source projects in GitHub, and having to continuously switch our development environment from one source control system to the other was not practical.
Other than that, we don't really think that Git is better than Mercurial (or the other way around). Git won by a narrow margin when we reached the "only one survives" point in our discussions on this subject.