Why cobalt reacts favorably with nitrogen?
According to the recent first principles ab initio DFT calculations done with VASP of structural, electronic and mechanical properties of 3d transition metals mononitrides $\ce{MN}$ carried out by Liu et al. [1], there is a local maximum of cohesion energy $E_\mathrm{coh}$ at cobalt for all possible/discovered structure types:

Ultra-soft Vanderbilt pseudo-potentials (US-PP) … as supplied by Kresse and Hafner …, with local density approximation (LDA) … and PW91 general gradient approximation (GGA) … were both used. It is known that LDA underestimates the lattice constant and overestimates the elastic constants and cohesive energy, while PW91 GGA does the opposite for the lattice constant and elastic constants by a smaller amount and yields more accurate values of cohesive energy.

Figure 1.
Cohesive energy per atom of the nitrides $(\ce{MN})$ versus their corresponding 3d transition metals $(\ce{M}).$
The three structures are zincblende (zb, black square), rocksalt (rs, red circle) and
cesium chloride (cc, blue triangle).
Experimental values for the rocksalt structure (green ×) … are also presented.

Since we are discussing synthesis from the elements, it's especially handy to use cohesion energy since for solids it signifies the energy required to disassemble the structure into an array of neutral free atoms.
As a rule of thumb, the higher the atomic cohesion energy is, the more stable the solid is.
Numerical values of $E_\mathrm{coh}^\mathrm{GGA}$ for cobalt nitrides are always exceeding the ones for iron, nickel, copper and zinc, and also $\ce{CoN}$ demonstrates mechanical stability across all structure types:





There is still a lot of controversy between the studies of preferred phase and magnetism [2]; I'm not sure though it is currently possible to interpret/rationalize or let alone predict this behavior by other, simplified means.
References

Liu, Z. T. Y.; Zhou, X.; Khare, S. V.; Gall, D. Structural, Mechanical and Electronic Properties of 3d Transition Metal Nitrides in Cubic Zincblende, Rocksalt and Cesium Chloride Structures: A First-Principles Investigation. J. Phys.: Condens. Matter 2013, 26 (2), 025404. DOI: 10.1088/0953-8984/26/2/025404.
Soni, H. R.; Mankad, V.; Gupta, S. K.; Jha, P. K. A First Principles Calculations of Structural, Electronic, Magnetic and Dynamical Properties of Mononitrides $\ce{FeN}$ and $\ce{CoN}.$ Journal of Alloys and Compounds 2012, 522, 106–113. DOI: 10.1016/j.jallcom.2012.01.100.