I think that the main reason is that the inhabitants of these media organs have all been influenced and educated the same way and honestly believe that that their target audience think like they do...which they don't. They get very mardy usually when they realise that they are actually a minority of crazies with a little temporary power.That's not why it's so widespread and obvious. The is no such thing as an integrated, co-ordinated and homogenous advertising industry. It is a tapestry of many agencies, each of whom have thought, "We'd better get some black people and gays etc. in our ads so that we don't appear *ist/*phobic." The seemingly overwhelming result is due to all the components being aligned the same way at the same time for the same reason. TV shows are the same. That's why there a suddenly so many BAME/LGBTQ+ main characters such as senior cops, barristers and inhabitants of soap opera communities.
That explanation is probably a SOBO, but that's the way it is.