They did not hop, even though the man had asked them to "hop in," because hopping is something done in the cheerful moments of one's life. A plumber might hop, for instance, if she finally fixed a particularly difficult leak in someone's shower. A sculptor would hop if his sculpture of four basset hounds playing cards was finally finished. And I would hop like nobody has ever hopped before, if I could somehow go back to that terrible Thursday, and stop Beatrice from attending that afternoon tea where she met Esmé Squalor for the first time.
But Violet, Klaus and Sunny did not hop, because they were not plumbers fixing leaks, or sculptors finishing works of art, or authors magically erasing a series of unfortunate events.