More likely, because all the current MacBooks are tightly designed around the dimensions of the butterfly keyboard, they needed to exhaust all the possibilities for "fixing" the feel and reliability of the butterfly design before backtracking to a (thicker) scissor mechanism, which would entail substantially changing the design of
5 different MacBook models. I'm sure tooling up to produce those Unibody cases isn't cheap and that they budgeted for 3-4 years between major re-designs.
...also, they probably don't want to change the MacBook range case design until Intel
finally get the 10nm chips out of the door, so they can take advantage the new (hopefully cooler-running) chips... otherwise they might have to make the new MBPs
thicker to accommodate the keyboard... oh, the humanity!
(NB: I just hope they fix the Intel MacBook range
before any hypothetical switch to ARM so that there's still a credible x86 option during the transition and people aren't forced to switch - or jump ship - on day 1).