It is up to the publishers, so you should contact them and request support. Remember that they will incur support costs by adding a platform so it may not be as simple a decision as it looks.
What about moving DOS emulation feature to Steam itself? It would be up to Steam devs to ensure that right dosbox version is available (which doesn't seem to be a complicated task), and publishers would redirect to Steam support on dosbox-specific issues.
There can also be other multi-platform runtimes - e. g., Flash.
Nothing extracted yet.
Many games available through the steam library are made so using of the DOSBox emulator on Windows (the original Doom games for example). Considering the availability of this particular software on Linux, it would be good to see those games released on the Linux client.
If it's up to the publishers, perhaps give them a reminder as this should bolster the list available games and provide more incentive to people like myself who enjoy the older games frequently.
Thanks for the work so far.