You're forgetting a "little" problem: Sigma have limited manufacturing capacity, and their lines for full frame are pretty much maxed out just with the current volume. The same goes for Tamron, their business for E-mount and Z-mount alone is brisk enough that they have little spare capacity to produce lenses for other mounts. Well, they do a ton of OEM work too, but we don't know exactly which models they make under that arrangement.
Sigma's APS line, however, is mature and has probably seen a stable amount of sales for years now. Which means that introducing a new mount into that mix is much easier, especially when there's a big market starved for better optics. Just the fact that they're moving pretty aggressively, and that all of their APS catalog will be ported by next year should be causes for optimism. Nikon, who are most definitely selecting each new introduction on a case by case basis, haven't allowed Sigma to release most of their crop lenses, much less any full frame ones. Canon might be more generous, who knows.
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