Digital ICs and analog ICs have different requirements for optimizing performance. Digital ICs are mainly concerned with switching speed and power. Analog ICs are usually more concerned about noise, transistor gain, ac frequency response, etc. Thus the the IC manufacturing processes are optimized for the particular needs of the type of circuit being made.
You can build digital circuits with an analog process or vice versa but the circuit performance generally will not be optimum. For mixed signal design with both on the same chip they do the best they can to optimize both but probably emphasize the analog requirements since that's usually more critical.