Because no party in the assembly has the support of the majority of the Punjab Assembly.
The PML-N is a minority in the assembly - it will need to get another party to partner with it before the Assembly can be called to session.
Daily Times - Leading News Resource of Pakistan
A session of the Punjab Assembly will only be called when a party will have the support of majority of members, Governor Salmaan Taseer said in a press conference on Sunday. “No political party has a clear majority in the assembly,” he said. “A majority will be formed with the coalition of at least two political parties.”