Canto 2 -
Ayodhyā-kāṇḍa
Chapter 20: Kausalyā’s Lamentation
Text 2.20.26
स स्वभावविनीतश्च गौरवाच्च तथानतः।
प्रस्थितो दण्डकारण्यमाप्रष्टुमुपचक्रमे॥
sa svabhāva-vinītaś ca gauravāc ca tathānataḥ
prasthito daṇḍakāraṇyam āpraṣṭum upacakrame
saḥ = Rāma; svabhāva-vinītaḥ ca = was humble by nature; gauravāt = out of respect [for His mother]; ca = and; tathā = then; ānataḥ = submissive; prasthitaḥ = to depart; daṇḍaka-araṇyam = for Daṇḍakāraṇya; āpraṣṭum = to take permission from her; upacakrame = He prepared Himself.
Rāma was humble by nature and submissive out of respect [for His mother]. He then prepared Himself to take permission from her to depart for Daṇḍakāraṇya.
This explains why Lord Rāma did not sit on the seat. What He told His mother is set out from the next verse.