A combination of factors really. I've had such huge value for money out of the EasyPIC4, most of the time these days if I need a special test signal etc or special measurement etc for the workshop I'll make it by programming the EP4 and carrying it out to the workshop. So it gets used as a piece of test equipment.
I wanted to start doing apps with the touchpanel and GLCD, interactive icons and virtual pot sliders, bargraphs, on the screen etc. That's some nice stuff! My EP4 doesn't support the touchpanel.
The EP6 has the built in COG text LCD, so this is a step up. Most of my apps these days use text LCD for the application, with the COG LCD I can have that AS WELL as the application LCD. So I can use it for debugging etc. I don't use "proper" debugging because it is too limiting, i do fast real world apps (like power control and signal generation etc) so having the second LCD I can display what ever readings I need without affecting the main application LCD. I've done that in the past by using a 4x20 LCD and using the top 2 lines as app display and the bottom 2 lines as debugging display. having 2 displays is better still.
And the other reason, because with 2 dev boards both with LCD I can develop master/slave apps much easier. Even though i've been wriitng Windows C apps for years it's limiting with dev board -> PC comms. Having 2 dev boards and being able to set master code and slave code and test both working together is a big bonus.
So my EP6 will be used now as my main dev board and the EP4 will be used for signal generation and comms etc to support the EP6.