Here’s my uneducated guess on the subject.

There is only so much reserve equipment that the utility services can keep in stock. So it doesn’t really matter where the first wave of attacks hit, as long as it does extensive damage that draws down the stocked up supplies. Russia is forced to use up the stock to get the power back on to the affected regions.

Then hitting the better defended locations in the Moscow/St. Petersburg become a several week/months blackout instead of a minor inconvenience of a few hours or days.

If the regions hit are also areas for refineries and equipment manufacturing for the war it’s a continuation of infrastructure degradation not terror bombing like the Russians are doing.