Because game design. Every option needs pros and cons. Coal's downside was modeled in high levels of particulate pollution. Coal's Kilodeaths are actually modeled in that pollution drives down population. Nuclear in the game should also model real-world risks, and catastrophic meltdown is such a risk. There need to be meaningful pros and cons to each choice, that are fairly transparent to the player: "Oh I built too many fission plants, therefore I increased my chances of a meltdown". Coal plants kill lots of people gradually, nuclear plants are cleaner, but have a possibility of a catastrophic event. That's actually realistic.
Also, to be meaningful for gameplay, things in game do actually have to be condensed. This includes: time, distance, numbers of people, and chances of things happening. e.g. if the real-world average number of meltdowns per uranium reactor per year is 1/30000, then it needs to be like 1/100 in a game of SimCity for the choice to have a meaningful effect on player choice, and to represent how that decision affects the situation in the real-world. Basically, because simulation games are very granular and time-compressed compared to the real-world, very small actual chances need to be amplified to become meaningful game choices.
There are about 450 nuclear power plants in the world today, and there have been 99 nuclear power accidents since the 1950s. Sure, not all of them were actual Three mile islands, or Chernobyls, or Fukushimas, but for gameplay purposes to represent the effects of choices on the city, it would be reasonable if each nuclear plant had about a 25% chance of meltdown in 50 years.