Is it going to be a surface lot or a multi-story lot?
I think this site probably needs to be developed, but there is some long-term planning here. Metrolinx needs to hold on to land near stations for future use, in this case, they may eventually need a new Bus terminal or LRT portal. GO is also still primarily a drive-up commuter service, and will be until they can improve frequency and fare integration. If they keep this as a surface lot, they will be able to develop in the future once they are confident they won't need the land, and transit access is sufficient to not need the parking.
Currently, they may not have a set plan for the land, so a surface lot is a good intermediate use while a plan is developed. If they sell it for housing development and then need it in 10 years, the cost of whatever project needed the land will end up being much higher.