The problem is figuring out how big the event is, and how directly it will hit us. So while we have over 17 hours to prepare, there might be some false positives due to our limited prediction skills. And, no matter the real consequences, people have limited patience for large economic disruptions over things that turned out to be nothing.
Even better, if you turn off the electricity and prevent major destruction and nothing happens (other than the power down/up) then you're the one who caused "the problem".
There's no reward for fixing a problem that doesn't happen and that people don't want to believe even exists. Bonus, if other networks are damaged while yours aren't, it must be because you protected your network so you're responsible!
17 hours sounds like a long time, but it's not like there's a big red button and a guy standing by to press it.
So the information chain, starting with the telescope that detects the flare, and then has to work it's way up the food chain, so that sufficient people agree, and take presumably synchronised action, well, good luck with that.
You'd also ideally need a multi-hour warning, planes gotta land etc.
Make no mistake, shutting it down will result in some deaths [1]. And those deaths will be on the news tomorrow (if indeed news still exists.) On the other hand not shutting down will cause more deaths and massive destruction.
[1] think hospitals where the backup power failed, or didn't last long enough. Traffic intersections. Elevators. Water pumps. Airplanes. Trains. Take your pick.
In https://en.wikipedia.org/wiki/Carrington_Event it is estimated that the time from spotting the flare to the solar storm was 17.6 hours. That's plenty of time.
The problem is figuring out how big the event is, and how directly it will hit us. So while we have over 17 hours to prepare, there might be some false positives due to our limited prediction skills. And, no matter the real consequences, people have limited patience for large economic disruptions over things that turned out to be nothing.