Why would hexacyanidocobaltate(II) be uncommon while pentacyanidoaquacobaltate(II) is common?
In Inorganic Syntheses (1946), a synthesis of $\ce{K3[Co(CN)6]}$ is described, which boils down to the following three equations:[1]


  $$\begin{align}\ce{CoCl2 (aq) + 2 KCN (aq) &amp;-&gt; Co(CN)2 v + 2 KCl (aq)}\tag{1}\\
\ce{Co(CN)2 + 4 KCN (aq) &amp;-&gt; K4[Co(CN)6] (aq)}\tag{2}\\
\ce{2 K4[Co(CN)6] (aq) + H2O &amp;-&gt; 2 K3[Co(CN)6] (aq) + 2 KOH (aq) + H2}\tag{3}\end{align}$$


The procedure requires precipitation of the cobalt(II) cyanide by carefully adding a $\ce{KCN}$ solution to a cobalt(II) solution. This precipitate is filtrated and washed, then redissolved in excess cyanide solution giving a deep-red solution. This in turn is heated to reflux for some 15 minutes, whereupon the colour changes to yellow indicating the formation of the hexacyanidocobaltate(III).[1]

I personally doubt equation $(3)$. Instead, I would assume that the oxidation of hexacyanidocobaltate(II) to the corresponding cobaltate(III) proceeds unter oxygen consumption, not hydrogen liberation. That would give equation $(\text{3'})$:

$$\ce{4 K4[Co(CN)6] + O2 + 2 H2O -&gt; 4 K3[Co(CN)6] + 4 KOH}\tag{3'}$$

In any case, however, the presence of $\ce{[Co(CN)6]^4-}$ or hexacyanidocobaltate(II) as a stable intermediate in solution suggests to me that your premise is false and thus the book you cite from incorrect.



Reference:

J. H. Bigelow and J. C. Bailar (1946) Potassium Hexacyanocobaltate(III), in: Inorganic Syntheses, Volume 2 (ed W. C. Fernelius), John Wiley &amp; Sons, Inc., Hoboken, NJ, USA. DOI: 10.1002/9780470132333.ch72.