At the time (early 1970s), Pickering Township was in the middle of nowhere - Markham was a small town at the corner of Highways 7 and 48, Pickering as we now know it wasn't much.
Pickering was rejected early beacuse there would be some interferance with Pearson's aerospace, but also becayse the terrain was too hilly and most of it is prime class A agricultural land. It was brought back partly because the province wanted it to support its Toronto-Centred Region plan, shifting more growth east, and for other factors (Orangeville was too snowy and too far, for instance).
But even early on, there were forecasts that found that the government wouldn't have to even think about a second airport until at least 2000, but those where cast aside by bureaucrats and politicans who wanted to build a shiny new airport. The calculations they used to measure air travel grown assumed a constant rate of increase, though in reality, the increase started to level off.