Event Heirloom

Heirlooms for events like this are awarded for opening all the packs during the event. Afterwards, they would show up in the Mythic Shop to be unlocked with heirloom shards instead.

If you purchased the packs and had not opened them during the event, it would not qualify for this I'm afraid. It sounds like the event had already ended by the time you finished opening the packs, which would be the reason the heirloom was not granted.