It's not just the most efficient way, it's the only way to achieve this particular target orbit.
As the other answers have pointed out, an orbital inclination change must occur at the so-called ascending/descending nodes, which are the two points in the orbit at which the current and target orbital planes intersect. Anytime a spacecraft moves from one orbit to another, the original and target orbits always share at least one point in common - it's where the burn occurred. If you want to move from an inclined orbit to an equatorial orbit, the burn must occur at one of two loci where the planes intersect, both of which are above the equator. If you don't perform the burn where the orbits intersect, you will never magically jump the distance between the two, and you will never reach the target orbit.
Adjusting inclination to reach an equatorial orbit must occur when above the equator - it's not just the most efficient way to do it, it's the only way to do it. You can potentially increase the efficiency of your maneuver by minimizing the spacecraft's speed and the resulting delta-v change, which requires moving into a larger orbit, performing the inclination change, and returning to the original orbit. But even if you move into a wider orbit, the inclination change still occurs when over the equator.
You can adjust inclination at either the ascending or descending node, and it'll be more efficient to do so at whichever one has a higher orbital altitude, since the spacecraft's velocity will be lower. So, it's possible that the mission in question chose to perform the burn at the ascending or descending node specifically to maximize efficiency. But the fact that the maneuver occurred over the equator has nothing to do with efficiency at all - it is in fact required if you're moving to an equatorial orbit with zero inclination.
To sum up, one cannot enter equatorial orbit from anywhere except above the equator.