Price & Competition & Price....
One wonders how much budget is left for training or unplanned tech issues down the road. But no problem, its already good practice to 'postpone' the tech issues until back home
To me:
Chartering an Aircraft directly or via a Broker is like Russian Roulette !
Not saying there are no good Brokers on the Market, but it is just to appealing to take the cheapest offer and make the biggest profit.