^Because Gundam has never rehashed anything.
I mean, it's not like the Build series is meant to rehash from past Gundam series to sell Gunplas or anything, right?
Besides, the first Gundam Build is still the best of the Build bunch, Try was just a shitty harem anime disguised as a Gundam anime and single handedly ruined the series to the point where it they had to redo the series with Divers and Divers season 1 is just SAO crap but with Gundams.
It's going to take a miracle for this anime to not only set itself apart from last season but to get out of the writing sinkhole the series has gotten itself into and to erase Gundam Build season 1 from the fans minds.