Since an equatorial position has less room than an axial position, the larger tert-butyl group has a higher preference for an equatorial position.

The reactive tert-butyl group undergoes more extensive electron delocalization when it occupies an equatorial position than when it occupies an axial position. As a result of this, there is a higher preference for a tert-butyl group to occupy an equatorial position.

The tert-butyl group can form more favorable 1,3-diaxial interactions when it occupies an equatorial position. This causes a higher preference for a tert-butyl group to occupy an equatorial position.

Because a tert-butyl group is larger than a methyl group, steric interactions associated with an axial tert-butyl group are greater than those associated with an axial methyl group. As a result of this, there is a higher preference for a tert-butyl group to occupy an equatorial position.