I'd think there are lots of factors. If it weren't for the Canadian purchase requirements, I'd think that size wouldn't make that much difference. But because of it, it requires significant investment by anyone other than Bombardier, who have already bitten the bullet, and set up a production line in Thunder Bay (which essentially TTC is paying for in their order ... but with an original order of about 200 cars, with an option for I think 400 more).
I was looking at the specs. Looks like it's designed to open with about a 24-minute one-way trip, and a frequency of 3.25 minutes. If you assume a round trip, with terminal time of 55 minutes, then you need 17 train-sents. With a 120-metre train of 4 cars, and 25% spares, you need 85 cars ... increasing to 138 cars by 2031 when they talk of 2-minute frequencies.