There are several factors:

1. Cloud storage is likely to have a more resilient backup strategy;

2. Magnetic storms tend to be localized so you want to spread geographically. So if for example you copy data to hard drives and store them at home and work you're still in danger to a localized storm; and

3. And this is the big one. A single physical location exposes you to far more likely risks than solar storms such as fire, flooding and theft.

About #2, solar storms are world-wide.

The upside at least there, is that there is some warning; you could stick everything you care about into a Faraday cage. Power lines... not so much.

No need. Just unplug them from the power lines and you are good.

If you have that kind of concern for your data, there are solutions which involve actual nuclear bunkers... that would do the trick.