Simple reason behind the lack of replacements is because they are behind schedule.
It was meant to happen 2 months ago but got delayed further. Domestic 737 was due to start late November.
Tiger will have a net increase in aircraft once all A320s have been replaced, +3 Bali Machines.
Looks like they are fighting to get 737 Domestic up and going before the A320s go to Bali otherwise it's going to screw the Domestic schedule.