Joking aside... they don't talk too much about the development communities of the two systems, and it's more than likely that they will have to 1) hack the system to work with their infrastructure, and 2) may want to feed some stuff back, and perhaps hg is easier in that sense, being in Python. Also, while I don't know this, perhaps the authors are easier to work with than Linus, who is a very busy dude and extremely ... let's say ... sure of himself (which is mostly deserved).