Why doesn't WotC use established keywords on all new cards?
Landfall isn't an established keyword since it's not a keyword; it doesn't have its own article in the Comprehensive Rules, rule 702. Rather, it is an ability word which does nothing more than link a group of cards with a common theme/mechanic together.


  207.2c An ability word appears in italics at the beginning of some abilities. Ability words are similar to keywords in that they tie together cards that have similar functionality, but they have no special rules meaning and no individual entries in the Comprehensive Rules. The ability words are addendum, battalion, bloodrush, channel, chroma, cohort, constellation, converge, council's dilemma, delirium, domain, eminence, enrage, fateful hour, ferocious, formidable, grandeur, hellbent, heroic, imprint, inspired, join forces, kinship, landfall, lieutenant, metalcraft, morbid, parley, radiance, raid, rally, revolt, spell mastery, strive, sweep, tempting offer, threshold, undergrowth, and will of the council.


(the list might be updated when a new set comes out)

Having said that, it looks like Evolution Sage is the only card in that set with  landfall a landfall-like ability. Having only one card with that mechanic is (apparently) not enough to return a key- or ability word from a previous block; they generally only do so for evergreen keywords like lifelink, vigilance or shroud.