Every mobile tower has only a certain number of connections. When that limit has been reached, phones will try to connect to another tower further away, if available. Most eftpos terminals are WiFi now as it is a lot cheaper than dedicated land lines. They are competing for the same connections as everyone in the building who is texting, updating Facebook, live chatting, steaming etc.
The large AFL venues, such as Adelaide Oval, MCG, etc have upgraded WiFi capacity, either WiFi through the mobile network or dedicated "Free WiFi" on site. These stadium are used all year round so the investment by the Mobile companies is recouped.
Adelaide Arena is in the middle of suburbia and only for the 14 home games plus a few finals has the numbers there to stretch the mobile providers. Any upgrade done by the Mobile providers would not get a meaningful return on their investment, hence the struggle to get a connection at times with large crowds.
There is an article published in The Advertiser on February 25 2015 which discusses this issue and how it is being resolved at Adelaide Oval and interstate venues.