Upanishad talks exclusively about Emperuman alone; other parts of the Vedas talk about a demi-god first and then talk about Bhagavan as an in dweller of that demi-god. As those who perform Thirumanjanam are required to recite a shloka, they recite Upanishad as it is about Emperuman. Svami Desikan himself has said that this is auspicious. Hence, we recite Upanishad.