Probably because Second Life is a shit hole and has a reputation to be primarily visited by furries, *********s, and sex-aholics. Plus, Minecraft is a game while Second Life is more of a simulation/role play tool. Minecraft is played by gamers, and this would be something for gamers.
Minecraft has been used in the past to generate tens of thousands of dollars for charity, too. So it wouldn't be a new concept.
Hell, you could put up a server, invite people to come play on it for 24 hours or something, stream it live and have everyone in a TS server, and ask for donations to go towards the victims families.





