The link below will take you to many publications on this topic. The third book listed looks especially helpful. It's worth remembering that many of the books you can find through a Google Books search will themselves contain bibliographies pointing in the direction of still more books. In fact, you may want to search the books listed below for the word "Bibliography" and see what turns up. Good luck in your research!
The Indian sub-continent has a few natural entrance points. Over half the area that lies at the bottom is protected by the Indian Ocean and the Himalayan mountain range on the top was quite difficult to cross in the past for a large part of the year. This allowed entrance either using ships or an army could enter from a small area on the North-West. This posed as an impediment both for Mughals who wanted to enter into the region and the Guptas who wanted to spread their dominion outside.
Geography is always a key factor that can help or hinder empire building, and is a pivotal challenge for any would be Napoleons. In this specific case, the vast territory of the Indian subcontinent because of its size is something that presented an impediment to the two civilisations that you mention. Have a look at the following link for more information:
You might find it useful to use Google Books to search for more information about this topic.
The Indian subcontinent is peninsular in nature, extending toward the Indian Ocean and between the Arabian Sea and the Bay of Bengal. It is a diamond shaped South Asian region bordered by the Himalayas, Hindu Kush and Arakanese. It is home to about 34% of Asia’s population.