To increase the pixel count, the sensor has to be divided into smaller and smaller pixels.
The result is a decrease in sensitivity of the camera and an increase in noise because the amount of light collected by a single pixel is smaller. At the same time, increasing the number of pixels is supposed to lead to more details (resolution), but in order to achieve that better lenses with high resolution and a lower lateral chromatic abberation are needed. However better lenses are bigger and don’t fulfill the requirements for ’small’ cameras any more.