12 MP is the sweet spot for photography. The eye takes in 6 MP in one go, but when you are printing you have to 2x oversample, which means 12 MP. That’s enough for a billboard or for an A3 magazine spread.
The smaller the pixels, the less light they gather, which means the more noise. So, for the same generation of technology, a 24 MP camera has poorer low-light performance.
It’s a relatively simple matter to reduce the noise of a still photograph at 24 MP, but doing so for a video requires 25–30x as much computing power multiplied by the length of the video in seconds.* The work can be reduced, of course, by downsampling to 4K immediately, which is about 8 MP, but in doing so you lose a lot of your potential quality advantage.
12 MP provides a lot of processing options for creating 4K video, and the sweet spot for still photography. Going larger just means you lose low-light performance and increase processing time.