Because crypt fluctuates and steam does a lot of business.. the coins they would liquidate as they want USD would likely impact on the prices. They need to add the code and support for taking these purchases or use a third party in short its more work for them to support it.

Also if they take say 1 DGB in at current time and sell straight away chances are they will get less unless buy pressure is up so in areas they can lose money.

Steam runs their own economy with steam cash, credits or whatever you want to call them pegged against usd. Steam cards bought are in usd so they get what they want. Basically the only incentive for steam to support crypto is if it makes them money. Everyone has a credit card... they have no need. Then add on top you have to worry about anti laundering/terrorist support reporting. At present the credit card systems all do that for them. Again too much work for them.

We'd like it but its not going to happen anytime soon. valve make enough money and its no threat for them losing market share so no need.