this is why; that means; so it is that
A phrase used to sum up a reason or conclusion, often after an explanation. Equivalent to 'that's why' or 'so that's the situation'.
He worked late last night. That's why he overslept this morning.
In short, it means the plan has been canceled.
Kanji spelling using 訳; less common but seen in some writing.
Full kanji spelling with 言う; rare and somewhat stiff.
From と (quotative particle) + いう (to say) + わけ (reason, conclusion) + だ (copula). Literally 'it is the reason that one says', grammaticalized into a fixed expression.