By being on the equatorial positions, bulkier groups are allowed to remain as away as possible and relieve the molecule from torsoinal strain, which otherwise (if these are on axial postions) parts of these bulkier groups are thrown closer to each other and may increase the torsional strain experienced by the molecule, there by reducing the stability.

I'm agree with the others answers, but i want to add something: this stability in the ciclo compounds in the ecuatorial position is only with alkyl substituent (this is what i read in Wade et all 2006).

Grettings.

Having the t-butyl group in an axial orientation on cyclohexanes leads to substantial 1,3-diaxial interactions with hydrogens or other axially-oriented substituents. This steric strain has been approximated at ~11.4 kJ/mol per (CH3)3C - H interaction. Whereas, an axially oriented methyl group incurs substantially lower steric strain due to diaxial interactions with hydrogens (3.8 kJ/mol per CH3 - H interaction). Thus, a ring-flip that leads to the larger group being oriented equatorially is more energetically stable since the largest group now avoids these interactions.

Thank you all for your answers. But why this stability in the equatorial position is only with alkyl substituent?

Please do correct your question , b'coz theres no compound of this name, may you missed some thing in the name............after tertiary...

1,3-diaxial repulsions (Van der Waals repulsions) between the axial hydrogens located on C-3 and C-5, and alkyl group is very smaller when the bulk alkyl group (t-butyl) in equatorial position.

I'm sorry for making mistake in my question. What I thought was tertiary butyl. I got the appropriate answer. Thank you for correcting my mistake.