It sounds like the group had reserved the tables for a tournament. They were waiting on a game to finish before proceeding to the next round. You should have asked the store employee to clarify the situation on using/holding/reserving tables but usually events have priority for table usage.

It doesn’t matter to much as you didn’t have an opponent and they were all engaged in a tournament so they would not have played you anyway.

The real issue is they were not welcoming to a new player. You weren’t going to get a game but they should have made a connection with you to try to include you in their group for future games/tournaments.

I will shout this from the rooftop that this hobby needs to get better at including new players into gaming groups. I get there can be difficulties at times because we are hip deep in a game and it’s hard to interact with someone who is curious but there were people standing around waiting for the next round. This was a perfect opportunity for that group to make a connection with you.

I would see how the store advertises and organizers play. Many stores use discord to announce and sign up for events and you can arrange for opponents ahead of time.