The major currencies tend to fluctuate within a certain range (which quite gradually decreases over time).  Bitcoin, on the other hand, doesn't have any range - it could be $500-1000 one year and over $10,000 the next. 

To Steam, they look at this and even if they receive fiat money for a flat fee from BitPay (I doubt that they hold much or any of the BTC that they receive), they wonder what will happen to their customers if the range moves downwards rather than upwards.  Would they be afraid?  Would they blame Steam or complain to them?

You can imagine that it's a lot of uncertainty to them, especially when only a fairly small amount of their payments would be in BTC anyway.