Presumably it could if you factor in coal's carbon externalities.

What makes you think that?

Not sure why I can't edit my post. Anyway - downvoted for asking for clarification on an unsupported assertion. HN at its most scientifically rigorous!

Building a new plant is always going to be more expensive than just digging out some more coal and burning it in an existing facility.

Also - we have lots of coal.

The trouble is no one's ever built anything like this yet, so it's still risky. That's part of why we need government incentives. Once we know it works the incentives won't be necessary, the market will be happy to finance more of them.