Anniversary countdown art isn't a given; they posted countdown art from the original artists (less
Kuniharu Komiya) for the second anniversary, and year 3 posted the First Note special pages as a countdown and had art from the original artists (again, less Kuniharu Komiya) sequentially following the anniversary, but last year (year 4) the commemorative art was posted sequentially after the anniversary.

tl;dr: There's not a reason per se, it's just not a given that they do a preemptive countdown campaign. (Though as of ~45 minutes ago they have posted a teaser, but that's still not necessarily indicative that they're going to do a campaign before the anniv itself.)