Probably because people are switching away to get fairer deals from other suppliers. Don't listen to heir customers and raise bills and see what you get.
We stayed with them as we wanted to fix our bills and the longest fixed tariffs were from British Gas or NPower and, as I'd rather freeze than use NPower again, we didn't have any choice!
If you didn't know by now your being legally mugged by the big six energy companies ,I realised this when this country privatised the energy market it was doing what it's always done , created another cartel/monoploy.
Please if your one of these people who have never switched try these two to start with there are lots of small energy companies out there which will save you a small fortune ,but please i must stress do not set up a direct debit with them, My dad did and even they slammed on a extra charge (hey that's Britain for you)so if you can pay cash by quarterly bill .