Kshetriya is another name for body. And because he knows all bodies as also all good and bad acts, therefore, he, who is the soul of Yoga, is called by the name of Kshetrajna.
क्षेत्राणि हि शरीराणि बीजं चापि शुभाशुभम्। तानि वेत्ति स योगात्मा ततः क्षेत्रज्ञ उच्यते॥