Two more differences between a Yonge and a Bloor extension:
1) The Yonge subway only needs to be extended 4 km past the border before reaching the soon to be densest part of the Region. Furthermore, the entire path to that point is a growth corridor in and of itself. However, the Bloor subway needs to travel at least twice as far within Peel to reach MCC, and the most direct route is off grid, underneath subdivisions. You can be sure that if the focus of York Region's development was located at Leslie and 16th, a subway extension would be out of the question. Or, if MCC was located at Dixie and Dundas, I'm sure that the subway would already terminate there.
2) Travel patterns between York and Toronto are different than between Peel and Toronto. Due to numerous large employment and entertainment areas between Steeles and downtown, many York residents who use the TTC get off in midtown. Highway 7 to downtown might be a 40 minute trip, but it's not as though everyone is going to travel that far. However, I would guess that most Peel commuters take the TTC straight into downtwon because there's less in between. Peel is therefore better served by a GO train that doesn't stop anywhere in Toronto except downtown. York and Peel may generate the same number of TTC trips, however the nature of those trips is quite different.
I think you're on to something here. If I may add:
3) North York, to me at least, seems more visible than Etobicoke in the Toronto mindset. The Yonge corridor through North York contains probably the most successful suburban centre development in Toronto, if not the GTA. Etobicoke's city centre plan is still mostly just a plan as far as I know. If The Spadina extension, for most people I would wager, only makes sense because it serves York University. If there was a large university on the way to MCC, I'm sure there would be more urgency to extend the subway there. Yonge through North York is
the urban drag that has been the centre of development for a rather long time. In Etobicoke, the main urban road would probably be Lake Shore. Bloor (and Dundas for that matter) are pretty suburban, the dreaded Six Points interchange posing a huge problem.
4) Geographically, there are few if any obstacles between downtown and central York Region. Between downtown and Peel there is both the Humber River Valley and the Etobicoke Creek Valley. These are not obstacles to construction necessarily, but they do impact how we see the city-region. Both valleys limit the street grid connectivity across the west side of the city. Which leads to...
5) The boundary between York and Toronto is pretty arbitrary. Even small side streets cross the border. There are many options for motorists, pedestrians, transit agencies, etc. so they feel more connected to each other. There are relatively few connections between Toronto and Peel (not to mention across the Humber). The boundary is more logical and more tangible. Mississauga feels further away, even if it's not.
6) The added transfer to get downtown from the Bloor-Danforth line makes travelling feel longer. Additionally, inevitibly an extension to MCC would get a lot of people asking "why the hell doesn't the subway go all the way to STC?" A very valid question that the TCC and the City probably want to avoid answering ("there's no real reason").
If Mississauga ever wants to pursue a subway extension to MCC, they should contemplate planning a new urban centre along its route - Dixie Town Centre or something like that. Hinging the success of an urban centre on a subway extension has worked out very well for York Region - the problem for MCC being that it has been fairly successful
without a subway stop.