Taking a look here: http://www.publications.parliament.uk/pa/cm/cmfbusi/b01.htm. It seems like the next backbench session is about famine in Africa and human rights in Asia. The next session is a debate on whether MPs should be allowed to take 'handheld electronic devices' into the commons. Are this committee telling us they cannot shuffle their own timetable around to fit in e-petitions that make the magic 10k signatures?
As for 'there is no mechanism to get MPs to sponsor a petition' ... don't they know any other MPs? Surely it wouldn't be too hard to get someone to act as the sponsor?
I suspect they just don't support the 2 petitions that have been successful so far, at least not above their own pet causes.