When it proved hard to bootstrap that and seeing that the Steam Machine itself they started looking for options. Wine was already pretty good but DX11 support was bad. I do not know if it was serendipity that DXVK started showing promises around they same time they started looking into wine or whether it was the reason for them focusing into wine in the first place.
The rest is history, although the side effect is that it pretty much killed native ports for AAA games on Linux [2].
[1] for some values of native. Many, most, ports are based on internal close source equivalent of wine.
[2] I do not really care about native vs proton, but it would be nice if game companies did officially support proton.
It speaks more to me that they released Proton rather than shelving it after losing the original motive.
I know technically they're doing it to make money, but I can't help like feel it's also something of a labor of love as well. It would have been much easier to leave people in my (our?) position behind, so I appreciate the heck out of Valve for putting in the effort. I imagine they're going to have my goodwill for a long time as a result.