I think there could be a couple reasons. The first one is that they try to control the KIA/KIO and TNLA areas as much as they can before signing the nationwide ceasefire. If they can control more areas, they own them by the time signing the cease-fire. I think they will fight till the day of signing nationwide cease-fire. In this case, government and army have strategies to control ethnic areas, especially military and economic strategic areas as much as they can before signing nationwide cease-fire.
The second reason might be that government could not control the army. The Burma Army can do whatever it wants without government consents or approvals. In this case government and army have different objectives on peace process in Burma. The army may continue fighting after signing nationwide cease-fire.
The second reason is less likely because if government cannot control its army, it would not work hard for signing nationwide cease-fire. If there is still fighting after nationwide cease-fire agreement is signed, the government will lose its face to international community. The government doesn't want this happen since it tries to show international community that peace process in Burma is on track and working.
Posted by Pon Nya Mon