Why is the Heart Sutra so important in Mahayana Buddhism?
Maybe because Clinging/Upadana is such a powerful destructive force even advanced practitioners still have a tough time dealing with. So the non-clinging theme reflected thru insight into Emptiness/Sunyata throughout the sutra would be a great way to remind us about it. That's why in many Mahayana temples, this sutra is usually recited at the end of any session, after other sutras have been recited, sort of a reminder to the practitioner of the Clinging elephant in the room. Anyway, don't worry too much about why it's the most popular or most important. It's a pretty common stock phrase in Mahayana to say this or that sutra is the most popular or most important. You'll see the same thing being said in the Lotus Sutra, the Amitabha Sutras series, etc. Afterall the spirit of the sutra doesn't seems to deviate from the Buddha's teaching about the 3 characteristics inherent in all conditioned dhammas: impermanence/anicca, unsatisfactory/dukkha, and non-self/anatta. And the sutra simply reminds us that it indeed holds true for all "conditioned dhammas", including the Five Aggregates, the Twelve Ayatanas, the Eighteen Dhatus, the Twelve Nidanas, and even the Four Noble Truths!