In Australia it's because of two main issues:
1) Electricity providers charge both flat rate access fees, and per kWH usage, but the access fees don't truly cover the cost of the network. When usage goes down they are left short.
2) Many electricity providers took advantage of a government scheme to build excess capacity in to the network, and charge users for it. In some cases whole substations were built and sat around unused. Now that people are using less or going off grid, they are left with expensive underutilised capital.
The funny thing is, the more they try to claw back and/or raise fees, the easier it is to economically justify going off grid. They are shooting themselves in the foot.