If the NBL could afford to prop up teams and bail them out of trouble then we might have 12-14 teams playing now. The AFL could have let several clubs fold over the years but they have the money and resources to give them financial assistance to keep going.
Also, some NBL teams have died not because of financial problems, but because there is no one willing to buy the franchise. Brisbane Bullets folded because no one wanted to buy the licence off Groves. This has happened a few times in the A-League where there have been clubs for sale, but no buyers. But rather than letting them die, which the NBL did, the A-League took over the licence until a new owner was found (Perth Glory and Adelaide I think?) (this is the main reason why I personally don't like the privately owned system)
This even happens in the NBA with the New Orleans Hornets owned by the NBA last season. It's a huge financial burden for the league to take on, but if they have deep pockets, like the NBA and AFL do, they can support poor teams. NBL can't, and as a result teams fold too regularly and it gives a very poor perception of the league, even though the same problems exist in the AFL!