It is by far the biggest ticket sales of any sport at the Olympics - even 70% full stadia means about 58 matches with 30-40,000 odd crowds - even the athletics rarely sells that many tickets (usually across 18 sessions full to the brim, the equivalent of about 36 matches of 30-40,000.) and no sport other than track and field comes close. Moreover, it tends to create an easy win for 'spreading' the Olympic bounty, and rarely requires any meaningful new build, unlike Athletics. You are talking, when you add in VIp ticket sales and the usual venue add ons, generating revenue in the $200-300 million range - Revenue which, unlike most of the money from the Games (TV rights, sponsorship, branded merch), goes to the host organiser, not the IOC.
The thing about being an Olympic fanatic is that we often get here precisely because we rejected to some extent or other the overwhelming soccer culture that exists everywhere outside North america and the antipodes. So we aren't Olympic football's target audience. Casual fans who like football - of whom there are a LOT - are.
I do wonder, in all sincerity, if Brisbane might be better doing this with cricket than soccer - or if LA would have been better doing it with baseball.