the speech of the crowd is said to be "perfect babylonish jargon" for Rip. What is the main reason?

Rip hears the strange jargon of his new setting. True it is his old village but he recognizes little about it.

"The very village was altered; it was larger and more populous. There were rows of houses which he had never seen before, and those which had been his familiar haunts had disappeared." There were men outside-but none that he recognized. One man was speaking loudly about "rights of citizens-election-members of Congress-liberty-Bunker's Hill-heroes at '76-and other words", that were a perfect Babylonish jargon to the bewildered Van Winkle."

Babylon is a reference to the ancient Mesopotamia. The author means that all this strange talk of independence and rights meant nothing to Rip. He simply didn't understand. He was a fish out of water, so to speak.