Question: Why Krishna wears His Lotus feet mark on His forehead and why not Radharani?
Answer: A devotee of Krishna decorates the body because it is a temple of God.
The tilaka worn by devotees in the Hare Krishna movement indicates that we are in the disciplic line from Lord Chaitanya Mahaprabhu.
The upper part of this tilaka, shaped like the prongs of a tuning-fork, represents Lord Krishna’s footprint, and the leaf-shaped part on the nose represents a leaf of tulasi, Krishna’s favourite plant.
The two lines also represent the walls of a Radha Krishna temple, and so the space between the lines is Radha and Krishna’s abode. For other Vaisnavas the two lines may indicate Brahma and Siva, and the space between the abode of Vishnu.
In Krishna book at chapter 17 entitled “Lord Krishna’s Daily Activities”, Srila Prabhupad writes “Lord Krishna would immediately sit down and meditate on Himself. Krishna is Krishna Himself. Therefore, He was teaching us”.
Thus, Krishna is supreme personality of Godhead. So, He puts his own tilaka with His Lotus feet mark on His forehead as nobody is supreme to him.
Srimathi Radharani’s complexion is like molten gold(Tapta Kanchan Gaurangi).
Srimathi Radharani wears tilaka but it can not be seen because her complexion is of same colour of chandan tilak.
Thus, Tilaka remains hidden with her complexion.
Traditionally Indian women wore bindis on their foreheads, not the Gaudiya Vaisnava tilaka that we see today. That’s why we see Radharani and other deities of gopis not having the regular tilaka. Modern women do not put anything on their forehead. That’s probably the reason why Prabhupada told ISKCON women to wear the regular tilaka in order to distinguish them from others.
Another reason: It’s Her personal choice.