The changed media environment is certainly important.
I think the other factor to consider is the different political landscape. The Prime Ministers from Wilson to Brown were in an era when the two main parties were dominant. They commanded huge shares of the vote on high turnouts and had no real challengers. The parties were also strong social forces with memberships in the millions.
Now we are in fractured political landscape where the two parties are only polling a fraction of the vote they once achieved. Both face insurgent challengers, as well as a stronger Liberal Party and Nationalist parties in Scotland and Wales (which were almost moribund until devolution gave them a boost).
I don’t believe it is a coincidence that we have had a succession of weak Prime Ministers as the parties they represent have become weaker.