The youngest card type in Magic, and one of the most successful design experiments ever, planeswalkers, are subject to a lot of strictures on how they're made. That should be manifestly obvious - they all have the uniqueness rule, loyalty counters, and one activated ability per turn and only at sorcery speed built into them, for example, not to mention the rules about mythic rarity.
Still, in order for the cards representing the major heroes and villains of Magic not to get stale, their design has to change from the typical "plus ability, minus ability, ultimate" slate once in a while. But, according to Mark Rosewater, that change should be slow in coming.
When quincognito asked Rosewater on his Tumblr blog, "Is there a particular reason that static abilities on planeswalkers is design space that you want to hold off on for so long, compared to innovations like a four-ability PW, a transforming PW, or a planeswalker that becomes a creature which you've done already?" he answered:
I believe that planeswalkers have very limited design space, the smallest for any card type so I’m having us go through it slowly. The other things (save Garruk’s transform which was essential to it being a double faced card) have all still fit within activated abilities.
Things that could conceivably be planeswalker-specific static abilities, such as Vraska the Unseen's defensive creature destruction/personal No Mercy +1, are usually indeed implemented as their activated abilities. Still, a winged, Aven planeswalker, for example, could possibly have a static ability that always prevented creatures without flying from attacking it, or planeswalkers could even have protection or hexproof (although the latter is much smaller in scope given how few things actually target planeswalkers) and so on.