Speculation, but I think it makes sense:
http://blog.acooper.org/2009/12/speculation-the-real-villain...
tl;dr: Labor are using the filter as a political means to an end, to court the conservative Christian senator Steve Fielding's vote on the ETS (Fielding is more or less a climate change sceptic). Fielding, along with independent Nick Xenophon, hold disproportinate power in the Senate.