Actually, depending where you are, the season begins in late April to early May. (The first match of the season was April 17th - the first home match for a Canadian team was April 30th)
The A-League big wigs have stated that they don't want to compete with NFL and college football, so they wrap things up by the end of September. You can kinda see their point.
If you are a lucky fan (like those in Vancouver and Montreal), with preseason and playoff matches, you get 6 months of soccer. I'd like to see more, but I'm just glad I'm seeing any.