Perhaps a couple of years ago, in Western Ireland, I had the privilege to be in the company of a very great prelate just after he had offered a pontifical High Mass in the old rite.
Suddenly, quite out of the blue, he murmurred: "So beautiful, so beautiful. Why do they hate it so much?"
Afterwards, I started to recall the events which followed our entry into Full Communion with the See of S Peter. A determined effort was made to prevent my own admission to the presbyterate of the Latin Church. During those long, difficult, and extraordinarily painful months, I had the advice and support of some very good and holy men. I shall eternally be grateful to them. I remember all of the things that were said to me.
One of them said, and repeated it a number of of times, "John, you simply must realise how strongly these people feel about the 'Extraordinary Form'".
Another said he would explain to me why there was such prejudice against the old Mass. "It's because they associate it with a form of Catholicism which they think of as rigid, sin-obsessed, oppressive, and, frankly, frightening. They are afraid that, with the old Mass, the entire moral and cultural complex which they think they remember will return. And the thought terrifies them."
As the Bergoglians attack the Faith, it seems to me that the most insidious detail is their attempt to keep the Old Mass entirely out of normal parish life. But we need priests in parochial ministry who share the mind and methods of the great Fr Tim, once, so gloriously, of Blackfen. God forbid that the old Mass should be, or even appear to be, a precious ghetto for precious and exclusive clergy and for laity anxious to hide away from their fellow Catholics.
What is necessary is 'dual economy' parishes ... such as those often provided by the Oratories. An easy and gracious and unneurotic symbiosis ...
Joseph Ratzinger said in the 1990s when some English Catholic bishops were violently resisting a 'Corporate Solution' for Anglican Catholics: "What are they so afraid of?"
So ... To answer the question in my heading ... Fear. They hate the Old Rite because the Enemy has set fear in their hearts.
And, as C S Lewis once put it, our Foes are "those who have no joy."
Fear is his weapon of choice.