Why aren't government schools hiring teachers in all provinces?

Because so far the government does not allow it.

I don't think it is that the government will not allow it.  It is more that they can't manage it.  I did a back of the envelope computation once that told me that it would take over 8,000 teachers to put a native speaker in every classroom once a week from  grades 1-12.  This allowed for a quite massive national dropout rate from grades 5 to 6 when tuition kicks in.  It would be much higher if all children went through grade 12.  They can't really handle what they do in the big cities now and have to farm the management out to language centers.  Direct hiring is the ultimate solution but they can't or don't seem to try to manage it.  I have heard of one principal in Thu Duc who direct hired so it is possible.  Ultimately, it is the only way.

It is also partially but not entirely a matter of money.  As I have a friend who has two step daughters in the same middle school where he taught, we were able to compute the amounts that the school collected from parents above regular tuition to attend native speaker classes.  After all, he was paying it for them, even though they had a different teacher.  At 50 students per class, the number was about 4 X our pay.  Someone was making a lot more money than we were and I am fairly certain that not all was going to the language center that hired us.  Principals and district superintendents surely got in on the act.  The problem in the provinces would be that not enough parents would have the VND needed to sustain the graft that is being siphoned off by education bureaucrats in the big cities.