The British East India Company (BEIC) and the British government had a complex and tumultuous relationship with the Nawabs of Bengal, and there were several reasons why they frequently changed them. Here are some of the main reasons:

1. Financial control: The BEIC was interested in expanding its trade and commerce in Bengal, which was a significant producer of revenue for the Mughal Empire. By installing a puppet ruler, they could control the flow of revenue and extract more wealth from the region.
2. Strategic interests: Bengal was strategically located, connecting the Indian subcontinent to Southeast Asia and the Middle East. The British wanted to secure control over this region to expand their trade and influence in the region.
3. Fear of rebellion: The Nawabs of Bengal were known for their independence and sometimes even rebellious nature. The British feared that if they didn’t keep a close eye on them, they might stage a rebellion or form alliances with other powers to challenge British dominance.
4. Rivalries and factionalism: The Mughal Empire was already weakening, and the Nawabs of Bengal were often embroiled in internal conflicts, rivalries, and factionalism. The British saw an opportunity to exploit these divisions and install their own puppet rulers.
5. Debt and financial mismanagement: The Nawabs of Bengal were often deeply in debt, and the British used this as leverage to gain control over their finances. By installing a new ruler, they could restructure the debt and gain control over the revenue.
6. Subjugation and colonization: As the British East India Company’s power grew, they sought to extend their control over Bengal through diplomacy, military conquest, or both. They would often install puppet rulers who would pledge allegiance to the British Crown.
7. Pretext for military intervention: In some cases, the British used the excuse of misrule or corruption to justify military intervention in Bengal. This allowed them to occupy the region and impose their own administration.