NBA went from being popular in Australia (NBA Action on TV, cards, merchandise, ads on TV, Jordan, etc) to now suffering from a groundswell of patriotism that shuns something American like the NBA (shun the frickin' war, shun McDonalds instead - seriously!).
That's been the biggest issue, IMO. Timing of the season is a red herring, though the change itself might've lost a few auto automatically renewed and lost that habit.
Then from there, it's just been the stench of gradual decline. A lot of people would rather be associated with the top dog (AFL), the bold newcomer (A-League), or the solid regulars (cricket, swimming [yawn] and tennis), than be that "Going to the basketball tonight?" "Does Mark Davis still play?" "Err, no." guy.