Planeswalkers are Magic's central characters, and there is a huge ensemble cast of them - so, sort of paradoxically, we go a long time between seeing individual planeswalkers in the main block stories because others demand the limelight and new ones are constantly being introduced. Mark Rosewater addressed an issue related to this definitively on his Tumblr question-and-answer blog recently.
When coriell1 asked, "Do you think we could ever see a block with zero returning planewalkers? Some plane that's very removed from the areas of the multiverse we've seen?" Rosewater said:
Returning planeswalkers are always significantly more popular. We still make new ones but every block will have some returnees.
It's only natural that we like familiar characters we've already gotten attached to more, of course, but izzetreally took it a step further, saying "I’d honestly rather see some blocks with no new planeswalkers. We already have a lot of characters to follow!"