Microcontrollers Manufacturing Process
Finer processes require lower supply voltages, which tends to lead to lower core than I/O voltages, and multiple pins being required for power supplies. For example, 3.3V maximum on the I/O and 1.x V on the core. That's inconvenient in a small system. 

Also, a lot of the die is used up by the (fixed-size) I/O bonding pads and drivers, so making the rest of the chip finer does not yield as big returns in terms of reduced chip acreage. 

The mask sets are much cheaper for the coarser processes (maybe tens of thousands of dollars rather than ~$1M for a state-of-the art mask set), so it makes it more practical to have a wide range of different products. 

Microchip PIC products are actually produced in at least one fab (Gresham OR) that was previously used to make (at that time) state-of-the-art memory chips (Fujitsu).