During the peak era of Peshwa(Maratha), they were the most dominant power in India and had most of the area under their control. But they didn't control Delhi directly even after capturing it.
The Marathas had gained control of a considerable part of India in the intervening period (1707–1757). In 1758 they occupied Delhi, captured Lahore and drove out Timur Shah Durrani, the son and viceroy of the Afghan ruler, Ahmad Shah Abdali. This was the high-water mark of the Maratha expansion, where the boundaries of their empire extended in the north to the Indus and the Himalayas, and in the south nearly to the extremity of the peninsula. This territory was ruled through the Peshwa, who talked of placing his son Vishwasrao on the Mughal throne. However, Delhi still remained under the nominal control of Mughals, key Muslim intellectuals including Shah Waliullah and other Muslim clergy in India who were alarmed at these developments. In desperation they appealed to Ahmad Shah Abdali, the ruler of Afghanistan, to halt the threat.
Why didn't they place Maratha Emperor on Delhi throne and ruled directly? What was the rationale behind it?