At a guess:
Utd, the Milan clubs and Liverpool haven't been regulars in the CL. To get a decent (pot 2) ranking, the coefficient needed a boost, and this was a way to achieve that, offering a little to Chelsea, but nothing to any other English club.
That then left the need to get more people on board, which is probably why the Italians were invited in. As Barcelona/Bayern/Real are pretty much guaranteed pot 2 in perpetuity before this, they needed to sweeten the deal for them - bingo, more cash for former winners. I think the 4th automatic group place was rolled in with it at the same time.
More baffling is why anyone else agreed to it.