https://cartography-playground.gitlab.io/playgrounds/contour-lines-to-profile/
This cross line elevation profile shows on India south, only narrow area by the ocean is low flat, most of land on south is high plains.
High plains get less rain falls since the mountain stop the moisture air from ocean. Less rain falls means less vegetation, harder to grow farms, leads to less population.
https://transparentgov.net:3200/googlemaps110/default?layer_id=0&layer=Protected+Areas+Database+of+the+United+States+%28PAD-US%29+v3.0¢er_lat=17.770007730833278¢er_long=278.849321366226¢er_zoom=6&url=https%3A%2F%2Fgis.usgs.gov%2Fpadus%2Frest%2Fservices%2Fpadus3%2FFee_Managers%2FMapServer&overlayType=overlayType_image&panto=0&overlayOpacity=6&click_function=show_elevation_along_path&elevation_path_sample_count=40
India north elevation profile shows it is very low flat until hit the Tibet mountain. Rain falls from ocean can all the way reach the foot of Tibet mountain. Anually rain fall can reach 20000 mm, while southern califoria los angeles area is only 200 mm per year. That means some area in north india 1 day rain falls can reach 1 year rain fall in southern california.
Rain falls bring more vegetation, easy grow farms, make north india have more people than south india.