As far as coal's negative externalities go, carbon emissions and their impact on climate change were only some of the most well-known. There's extensive air and water pollution from both mining and electricity production, with negative health effects for everyone from the miners who mine the coal to those living near the power plants themselves. Mining operations can also have pretty significant environmental impacts in addition to the pollution. Fossil-fuel power plants result in thousands of premature deaths each year, millions of lost workdays, and billions in annual healthcare spending.[0][1][2] And estimates are difficult, so it's easy to undervalue their costs; for C02 emissions, for example, more evidence in recent years suggests that we're drastically underestimating the social and economic damages of carbon emissions.[3]
Very, very few of these consequences have ever been priced into coal-fired power production; for decades, it's been a de facto subsidy. And while we're seeing some shifts needed to lower emissions to fight climate change, many of the identified externalities still aren't priced in. Nuclear energy never received that kind of benefit of the doubt, or the sort of friendly regulatory capture that let society ignore many of its own negative externalities and risks.
internalized by regulation would imply that the externalities (eg. Climate change) are effectively factored into the cost of producing power by burning coal due to the cost of complying to imposed regulations.
It very much did happen for coal. That's what this whole global warming business is about.