In an octahedral complex, if we assume the metal ion to be inside a cube then ligands will approach from its faces. So the axial d orbitals will experience more electronic repulsion as compared to non-axial d orbitals. That's why they are higher in energy. But what I am finding difficult to understand is why non-axial d orbitals decrease their energy below the excited degenerate d orbitals level. Some answers that I found on other sites were mentioning about conservation of energy and that total decrease in t2g level and eg level are equal but still I cannot understand it.