In War of the Spark, we have the returning mechanic of proliferate. But we also have additional returning mechanics that don't use the associated keywords, namely Evolution Sage with landfall. This seems to happen a few times per set, where instead of printing the keyword they decide to explain the full mechanic on new cards. Is there a reason that WotC doesn't use established keywords with cards that use the associated mechanic?
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.
Keywords (and ability words) exist to telegraph the themes of the set
The primary purpose of keywords are to telegraph to players the mechanical themes of the set, and to tie those mechanics to the flavor identity of the set.
Looking at War of the Spark:
Amass is a build-around mechanic for blue, black, and red, and provides a flavorful tie to Bolas's Eternal Army.
Proliferate tells players that they can mechanically build around +1/+1 counters and planeswalkers, and thematically represents the teamwork of the Ravnicans (and planeswalkers) who are teaming up to fight Bolas.
Giving Evolution Sage the Landfall ability word would tell players that they should mechanically care about the lands they are playing. Which... isn't true? The only other cards that really care about lands are Nissa and Awaken Vitu-Ghazi, both of which are rare, and not at all the theme of the set.
Thematically, Landfall was used in Zendikar to represent exploring new lands and discovering new mysteries. Neither of which applies in WAR. Ravnica is thoroughly explored already, and if anything it's being destroyed. A mechanic tied to land destruction might be thematic (if a mechanical nightmare balance-wise), but landfall is a poor fit for the set.
Labeling Evolution Sage with landfall would make comprehension slightly easier (it's easier to remember the ability when you can categorize it as "Landfall - proliferate" rather than remembering the entire thing wholesale), but this effect would be primarily seen amongst experienced players (because they have to already know what landfall is to take advantage of the label), who a) don't need the extra help because they're experienced, and b) will categorize it as "landfall-proliferate" anyways, because c) the ability is extremely simple and doesn't require any memory aids.