It is due to the different nature of bending stress versus pure tension or compression. It also is usually only used for ultimate strength calculations and not limit or yield calcs. It takes advantage of the plastic deformation range to re-distribute the bending moment to the inner areas of the...
Roark and Young's "Formulas of Stress and Strain" has a good section on latticed beams, covering both the global and local buckling issues. Bruhn has a good section for calculating the properties of composite(meaning built up of several metal sections) beams, as does "Advanced Mechanics of...
I agree with trainguy. Each individual beam segment has its own moment of inertia, load, length, and boundary conditions. The full tower has a different moment,load, length, and B.C. This gives each a different Euler buckling value.