Likely because the line has never been designed to operate as such.apologies if this has been asked before, but is there a reason that Metrolinx isn't pursuing a phased delivery? this way they can say they've delivered part of the line at least while they sort out major issues (e.g. Eglinton)
Nevermind the whole transfer situation - if the line is to open piecemeal, than there needs to be ways to keep the different discrete sections isolated from each other, both in the sense of keeping the passengers out of the active work zones and vice versa, but also for things like power, signalling, data, etc. It doesn't seem like any of that was ever considered beyond the usual power isolating blocks.
And vehicles - they need to be cleaned, maintained, etc. Staff need a place to be able to prep for their day, store their own items, etc. The line in its current configuration only has one facility for that - so opening the east end first is a non-starter.