It gained no traction because it was more-or-less pointless.

Withdrawing all your coins from an exchange for one day, and then depositing them back the next day and thinking that that somehow makes them "safer" for the next 364 days is meaningless.

Only if you got a significant proportion of users (>50%) to engage in it would it be able to reveal exchanges which were insolvent or faking volumes, and that was never going to happen. If you can't get through to a newbie why leaving their coins on an exchange permanently is a terrible idea, then there is no way you are going to be able to convince them to take part in "proof of keys".

And as you say, even although some exchanges did delay or freeze withdrawals in the lead up to it, it made not a blind bit of difference to those exchanges. Instead of everyone saying "Hey, this exchange is probably insolvent" (Quadriga), they all just kept on using it and lost a combined millions of dollars worth of bitcoin a few months later.