Why? Can you explain this to me?
Because through Vatican II, he attempted to impose a non-Catholic religion upon Catholics. Through the Novus Ordo Missae, he attempted to impose a non-Catholic rite of worship upon the faithful.
In light of prior magisterial teaching regarding the Papacy and the Church's own indefectibility, Paul VI did things that were simply impossible for a true Roman Pontiff — protected as they are by the Holy Ghost with regards to the charism of the papal office — to do.