Valve explains why it no longer supports Steam

While this may seem like an easy solution, it is not in line with our beliefs about how to build healthy ecosystems. If there’s anything we believe in at Valve, it’s our belief that open systems are better in the long run, for ourselves and our customers. The openness of the PC ecosystem in particular enabled it to be a primary driver of hardware and software innovation, because anyone with an idea for a way to do something better was able to try it. When companies sell their hardware at below cost for a competitive advantage, or buy content exclusively for them, they are doing so to build a more closed system, one in which you can’t choose the software you want to use.

We don’t want that for PCs, and we don’t think you should either. You shouldn’t feel like you have to buy Valve hardware; You should be able to view it as just one option alongside all your gaming devices, and choose the one that suits you best. This means you can select a device that fits your personal trade-offs around things like price, performance, form factor, peripheral support, and everything else that matters to you. This is the power of the open PC platform, and hardware support goes against it.