I'm not sure that I can speak to the motivations of different governments over time, but I can say that the tax credit is designed as additional or extraordinary support to the mining sector that supplements the ongoing permanent support available through flow-through shares to finance their exploration spending. The credit is an additional credit, on top of the deduction, that investors receive from a mining company. I think governments over time have wanted the opportunity to review current conditions in the sector to determine whether they merit extending this supplementary support.
On May 10th, 2016. See this statement in context.