Worship of Skanda seems to be less popular in north of India , while he is revered more in Tamil Nadu , Karnataka and Maharashtra. What could be the reason behind this?
Because Lord Kartikeya left Kailash mountain (situated in North) and made his abode in Kraunch mountain (situated in South India) and spent lot of his time here. Read the complete story from Shiva Purana.
So few stories are more popular in South India than in North India.
However, he is worshipped in North too. In every Shiva temple, Kartikeya is also worshipped.