It turns out that a very real culprit may just be regulation costs.
“Things like how the game is rated can affect the price,” claims Mark. “If a game in Australia must be rated MA15+, whereas overseas it can be sold with a PG rating, then the price is likely to be higher in Australia because the potential market is smaller – a lower rating makes the game available to more potential buyers.”
http://www.kotaku.com.au/2010/11/why-do-videogames-cost-so-much/
the article points out several other possible/likely culprits, and chances are that at least a few of them are working together to cause the prices you see. But when it comes to downloaded software, the quote above seems to be the only one that applies. Other than, of course, "because they can".
Its also possible that they were forced to increase the steam price for political reasons. I have to imagine that the hard copy retailers were not too thrilled to find one of their competitors was able to sell the game for $30-$40 less than them.
I remember hearing a story about retailers in the UK boycotting games that were sold by steam, because it takes so much of their business, and that was for games that steam sells for the same price.