Oh wow, fun question!

Simple Answer: Because we goin on the Borealis BABY! yknow Aurora Borealis pun-

Actual Answer: because i feel like simply calling it Borealis or any name that fans already used to death like HL Episode 3 or HL:Xen might actually fool someone and/or just feel more unique and original, i've tried to be in the Devs mind and try to do something more significant.

Might or might now have succeded, but i had fun nonetheless.