I don't think this will ever happen, because it's all about ratios. The percentage of Canada-bound flights vs. total flights from US airports is far lower than the percentage of US-bound flights vs. total flights from many Canadian airports.

ORD would have to build a whole "Canada wing" if they had CBSA preclearance, which would tie up gates that could be used for flights to other destinations at off times. Gate space at ORD is already limited as it is. Same goes for LGA, which probably has more flights to Canada than any other US airport (just thinking off the top of my head...I could be wrong about that).