The reason Google and Apple are able to command such high commissions, it's because they are monopolies in their own markets. Most people who want iOS can only buy iPhones and use the app store. Most people who want to use Android, tend to buy Android phones and don't switch to iPhones.
As such, Google and Apple's app store's don't really have strong competition against each other. Plus, most developers pretty much have to build apps for both anyway. Their respective markets are huge enough.
Steam, on the other hand, although it had its own monopoly of sorts in terms of "central gaming repository", it's always had high competition from game developers releasing their games outside of Steam, and more recently GOG has become a direct competitor, and EA and a few other huge gaming companies started their own central repositories and keep their games off of Steam.
This is what forced Valve to make this move now. Competition virtually always favors the consumer (the companies, too, in the long term, as they are forced to improve their products, but that's another issue and most companies are too short-sighted to see that by themselves), and we're seeing yet another example of that.