Until very recently (see Front Street Hydro tunnel) there was really only 1 electrical grid feeding downtown. Getting ties into 2 independent feeds was basically impossible. Yes, TTC should have a backup for their own systems, but I doubt much thought was made for backups when the entirety of downtown was under a single point of failure (under Toronto Hydro's care).
The Front Street tunnel fixed that for Toronto Hydro. TTC's next electrical revamp (every 30 years?) will likely fix their end.
So, basically it's a matter of funding. In fact, it wouldn't surprise me if an accountant somewhere at the TTC had a line-item for it already; or to find it's been deferred or unfunded for a decade.