I think this property has to do with the ease with which cobalt makes complexes with $\ce{NH3}$ and amines. The variety and the stability of the complexes containing $\ce{Co^3+}$, surrounded by $\ce{NH3},$ ethylenediamine, $\ce{H2O},$ and/or $\ce{Cl},$ is unparalleled. Iron $\ce{Fe}$ does not make complexes with $\ce{NH3}$ under usual conditions. Nickel, copper and zinc do make one or two complexes with $\ce{NH3},$ but they are easily destroyed in acidic conditions. The variety and the stability of the cobalt complexes is unequaled.
The stability of the cobalt complexes is related to its electronic structure. The cobalt atom is argon + 9 electrons. $\ce{Co^3+}$ is argon + 6 electrons. 12 electrons are missing in $\ce{Co^3+}$ and needed to build up the electronic configuration of the next noble gas, krypton. If the $\ce{Co^3+}$ ion is surrounded by 6 ligands like $\ce{NH3},$ $\ce{Cl-}$ and $\ce{H2O},$ it "achieves its goal" of looking like a noble gas.
Cobalt may react with nitrogen to produce $\ce{CoN}$ made of $\ce{Co^3+}$ and $\ce{N^3-}$. And maybe this compound $\ce{CoN}$ is surrounded by six $\ce{N2}$ in the same way as $\ce{Co^3+}$ is surrounded by six ligands in the $\ce{Co^3+}$ complexes. This should be checked by X-ray analysis.
