What does the crystal field splitting diagram for trigonal planar complexes look like?
Say you put a ligand in the trigonal complex (or an equatorial ligand in a trigonal bipyramidal complex) on the $x$-axis, where $z$ is the "axial" axis.  Then that ligand is directly aligned with an $x^2-y^2$ lobe and falls on a "seam" of the $xy$ orbital.  But the other two ligands in the plane are closer to an $xy$ lobe ($15°$ off) than to an $x^2-y^2$ lobe ($30°$ off).  These effects balance so that both in-plane $d$ orbital see the same total metal-ligand intetaction.