The reason is "technicality" or more specifically delays in the paperwork and the prospect of more appeals and the finalization of the previous last ditch effort to claim that the firing squad was an inhumane form of execution.
These constant delays and constant reminders and the continued lack of remorse must be very frustrating for the families of those who lost loved ones and who are looking forward to the day that these murderers suffer the fate that they have been sentenced to. I can only try to imagine what this frustration and constant reminder must be like.
I guess there is always hope for the families that the government will keep its word and execute them in the near future.
On a slightly different note, Abu Bakar Ba'asyir, the alleged spiritual leader of Jemaah Islamiyah, was refused entry to the Batu Prison on Nusakambangan. Unfortunately for Ba'asyir he did not have the necessary paperwork to visit prisoners on death row and the guards told him to go away and come back when he was properly documented. The proper paperwork is the necessary stamps from the relevant offices of the Department of Law and Human Rights.
This did not make much of an impression on Ba'asyir or the members of the Ba'asyir entourage. However, there appears to be no such restrictions with regards to visiting regular inmates. So, he made the trip to Nusakambangan and visited those convicted of terrorism offences but not on death row.