^^ Who knows ... just pure speculation ... maybe because in the last 10 years they have developed a whole new and better modeling engine better than Helix / HX and it needs new hardware to run optimally and for the next 5 - 10 years of updating ???????
Helix / HX may not be going anywhere for now .... but there is no way in the universe [i.m.h.o] that the current Helix / HX tech and algorithms will be their top tier product in 2 - 3 years.
Ok, but even if they develop "a whole new and better modeling engine" and have new hardware to run it on, Line 6 is going to be modeling the same amps, cabs and pedals in Helix because they own them.