TLDR: this wet weather mode stuff is FIA mandated because it is basically a rule enforcement around traction control. The loss of traction was related to a weird interaction in low-revving engines, turbos not kicking in, and the rest of the systems trying to do something that anti-traction control stuff would prevent.

During the Sky commentary, Anthony Davidson explained the specific electro-mechanical interactions based on what teams were telling him.

The FIA absolutely do need the ability to enforce regulations, but perhaps they should be monitoring for traction control and then stewarding that, rather than managing parts of the ECU firmware...