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In large part because she asked them to do so.
Almost no "king" would do this. But Maria Theresa was a woman, and a young, attractive, one at that. So she turned her woman's "weakness" into a strength.
Her father had paved the way 17 years earlier, by making large land grants to Hungarian nobles for signing the so-called Pragmatic Sanction. But a large part of (unexpected) support at the time came from their love of this branch of the royal family. (They had rebelled against one of Maria Theresa's uncles.) Maria Theresa's address reminded them of that.