Answer:

The fact is, in 1757 they were not Mughals who were defeated by Marathas. Mughals were defeated years before this period. And not single time but almost half a dozen times Marathas defeated mughals.

Delhi was attacked by number of rulers before. All these rulers looted delhi heavily. Marathas defeated rest of mughals in past 40 years. Now Mughals were much weaker.

In 1752, Ahmad Shah Abdali, Prince of Afghan Durrani Empire looted Delhi very badly. He even ENTERED ahead to Mathura and Agra and looted much more than previous raids.

By this time, Marathas were greatest power in India. Hence Mughals asked Marathas to help them against Ahmad Shah Abdali in exchange of taxes of half of Mughal land. Hence in 1752 Raghunath Rao Peshwa went to his first north campaign. And Marathas successfully defeated AFGHANS and won some part of PUNJAB.

Here all Known that Marathas were real Power in India Now. Hence, the Mughals signed AHAMADIYA TREATY with Marathas. By this treaty it was decided that Mughals will give regular tax to Marathas and Marathas will protect mughals whenever any external invaders come.

Thinking practically, now Mughals were the puppets of Marathas so their was no need to send much force to keep north. Now Mughals were the allies who paid taxes for their protection.

Hence in 1757 Ahmad Shah Abdali again invaded Delhi and now this time he won Delhi. And kept Najib khan Udaula as his general to keep Delhi to Afghans. Hence remembering Treaty of Ahamadia Marathas went to Delhi again under the Command of Raghunath Rao Peshwa.

Raghunathrao defeated Najib Khan in Battle of Delhi in 1757. And this time Maratha army entered into Punjab very DEEP inside. They won parts till Attok and LAHORE. Son of Ahmad Shah Durrani ran from Lahore. And this region was handed to Dattaji Shinde to control.