When you could pick up a F100 for a few million US$, why would you buy a brand new jet with underslung engines, less cargo etc. (eg EMB190), massive lease costs or major expenditure to buy outright???
717 can't compete in hot and high, plus they are such a heavier aircraft for not many more seats. Also an orphan with Boeing shutting down production.
There is not much in the 100 seat market out there to compete, perhaps as mines get larger and FIFO becomes more important we will see a shift to larger aircraft like A320s. But then again, I don't believe Strategic Aviation found much love in the mining community for their A320s. Otherwise they wouldn't be embarking on RPT.