With space heating, we don't need 21 Inc. Anyone in the market for space heating with Bitcoin mining, can go buy a used miner on ebay now. Thus if that was a big mass movement market, why hasn't it happened already?
It has. Poke around the mining forums and you see people mentioning constantly that keeping their home warm is part of the value proposition of mining for them.
21 apparently wants to scale out appliance and device mining to the mass market. That hasn't happened yet because no one has done it yet. The mass market isn't interested in (perhaps less than optimally efficient) mining rigs as heating units, but they'll be happy with an integrated plug-in unit that costs them less because it mines for the manufacturer.
Also, there no real tax burden to the manufacturer, mining revenue is taxed essentially the same as product or service revenue, especially if converted to fiat right away. That might well be part of the value-add to a subsidized-product model of mining.