No, that's not the reason. I'll eventually update to 3.x (I guess I have to at some time), but ATM, it doesn't seem mature enough to me to do it. In whatever version, LWJGL will remain the primary hardware renderer.
I try to release two official versions each year. This year, I'm obviously lacking behind because of my RPG game project. My goal was to have a fully playable, advanced beta by the end of the year and I'm very close to get there. I want to get this done, so I'm spending quite some time to it ATM. There will be another official jPCT and -AE release though and it won't support LWJGL 3.x for now.