You forget they did made huge cities in Gangetic plains also, have you heard of Greek description of Patliputra. Indus valley civilization started to expand eastward into Gangetic plains during 1900-1300BC and while many Indus Valley sites survived as late 1000BC but urbanization in Gangetic plains started so late. It was like suddenly at 1500BC, Indus valley civilization ceased to exists. It wasn't like Indus valley suddenly cease to exist in 1500BC and some hypothetical race called Aryans landed in India in 1500BC.
Many historians believe when Rigvedic Saraswati dried in 1900BC due an earthquake and led to exodus in the east and many abandoned their urban life to hardship that came with migration, the people kept the legend of Saraswati alive with the mythology that Saraswati went underground and joined the Sangam at Prayag.