The vast majority of the population doesn't live in Civic, so it's not a 15min 6km drive, for example, for me to get to EPIC it's roughly a 30min drive.
But the distance and the drive isn't the problem anyway, it's EPIC's remoteness from everything else that is the problem.
There's nothing at EPIC, no restaurants, no pubs, no hotels or places to stay, nothing, except a race track across the road, a Maccas and petrol station, and an industrial district up the road. It's also hard to get to it using public transport unless you are one of the roughly 5% of the population that lives along the tram line.
It's been shown time and time again that if you build stadiums in a city centre that is central and easy to get to (especially by public transport), that not only are the events better attended, but it's also significantly better for the local economy because it's easier for people to make a day/trip of it.
In other words if it's built in Civic more people will attend events at the stadium on average, they'll spend more money, and they won't only spend it at the stadium. More people from out of town will attend as well, because they'll be able to fly in, catch a cab/Uber into town, and then stay at a nice hotel within walking distance of the stadium, restaurants, bars, shopping centres, and other local tourist attractions.
It's the difference between going to Homebush and going to Melbourne, and a huge reason why attendance at sports games is so much higher in Melbourne.