My guess is they wanted to move to 32 bit, and PIC32 is the only 32 bit range of any significance available in DIP, which makes breadboarding easy and cheap. It also has a decent on-chip RC osc, so fewer parts needed on a breadboard.
That seems to be the most compelling reason. But abandoning the 8 bit MCU might be a little bit too early. It's also a good starting point for teaching MCUs. Possibly just a prejudice, I'm a little bit afraid that we'll get more bloatware developers.
The problem with teaching on small MCUs is you quickly hit limits that need working around. 8 bit C compilers often have various non-standardnesses to work around with limitations.
PIC32 with the Microchip tools isn't really much more complicated to get into, than 8 bit, and very easy to migrate to from 8 bit as many of the peripherals are very similar and the IDE and programmers are the same.
e.g. the linker by default is set to just work if you don't have an explicit linker script , until you start messing with bootloaders you don't even need to know it exists.
Similarly with a single #include <xc.h>, the compiler automatically pulls in the device-specific includes based on the IDE's part type setting.
This makes the process of getting up & running with a part very quick & easy.
The only fiddly bit comes when you start using peripherals & need to do the pin mapping stuff, but most modern MCUs have that in some form.