Cannot say as to why. Because for a long time the energy market was not really seen as a priority or as an issue.
It has only really become newsworthy, an issue, since the global collapse back in 2008 and the ensuing recession, long lag time of economic recovery, and the squeeze on home finances,the age of austerity and all of that. That's when people started examining in detail what comprises the cost of their monthly bill, noting the cost of "green taxes", and when people started looking into the relationships between the electricity generating firms and the energy retail firms, and, of course,when people really started looking at the way all the energy companies appeared to act in concert.
According to OfGEM, there is no evidence of a cartel, but the suspicion remains. As a regulator, OfGEMs powers need to be reviewed, as well. They appear pretty toothless, to me.