There is little English language coverage of this important issue, but AFP explains it as follows:
The congress has adopted a new roadmap and timetable, which allow for two scenarios. A general election is to be held at the end of the year if the constitutional body adopts a new charter within four months of its own election set for 20 February. But if the commission deems itself with 60 days incapable of completing the job, a Plan B allows for it to call for immediate presidential and legislative polls for a fresh period of 18 months.
While Karim Mezran of the Atlantic Council writes in his blog post entitled Deepening Polarization in Libya, No Agreement in Sight:
An emerging and worrying trend of political blocs within the General National Congress (GNC) forming alliances with certain militia groups—creating new and divisive power centers—threatens to derail the transition as these power centers prioritize self-interest over the collective good. The injection of weapons onto the political scene not only hampers substantive efforts to build a new Libyan state, but also emboldens criminal elements and political factions with parochial interests.Efforts to broker an agreement that would create an opening for nascent institutions overcome this stumbling block have now collapsed. The goal was to have every GNC member sign a pledge upholding these principles. It appears that the two major political blocs—the Muslim Brotherhood-affiliated Justice and Construction Party and Mahmoud Jibril’s National Forces Alliance (NFA)—agreed. Given the fragmented, multi-polarity that characterizes Libyan politics today, however, the proposed solution collapsed after its rejection by, most notably, the al-Wafa Islamist bloc led by Abdulwahab al-Qaid.Zintanis militias, which have so far reluctantly stood by the government, already pledged to side with the people if it takes an aggressive stance against the GNC. Meanwhile, Misratan militias, who withdrew from the capital amid widespread condemnation following the fatal clashes in Ghargour in November 2013, have promised to return to Tripoli if need be to defend the legislature and the revolution. Out of public view, Prime Minister Ali Zidan manipulates the factions gripping the legislature to maintain a hold on power, despite growing pressure from external forces urging a “no confidence” vote to oust his administration.
And the bottom line is in Mezran's words 'The legislature is banking on the fact that the constitutional committee cannot admit weakness, leaving the legislature in power for several more months.' It is impossible to know the motivations, but if this exists as the widespread view that is quite damaging for the legislatures already woeful credibility. One truly cannot predict if or when there will be an outpouring of public support for a mass upheaval to sweep away the current system.