Because of Cinnabarās frequency of appearance in the manga.
At the beginning they got more panels: we have Cinnabar-centered panels or pages in chap 2, 6, 10, 11, so roughly twice a year since the manga updates each month.Ā
But then we see Cinnabar again only 10 months later, in chap 21 and 22, then in chap 27&28 and once again 8 months later in chap 36 and 13 months after that in chap 49, their longest āhiatus.ā Then only three months later, in chap 52, but 9 months after that in chap 61 (well, 8 if you count the last panel of chap 60). Then thereās only one panel featuring Shinsha in chap 62, their official come back is in chap 65, three months later, and now in chap 70, after five months.
My estimate of one year was a little catastrophic. I didnāt do the math but, without taking the plot into consideration and only thinking about the frequency with which they tend to appear, it would be reasonable to expect them again in another 4 or 5 chapters, which is four of five months.