First, the Manchus had entered "China proper" because Dorgon responded decisively to Wu Sangui's appeal. Then, after capturing Beijing, instead of sacking the city as the rebels had done, Dorgon insisted, over the protests of other Manchu princes, on making it the dynastic capital and reappointing most Ming officials. Choosing Beijing as the capital had not been a straightforward decision, since no major Chinese dynasty had directly taken over its immediate predecessor's capital. Keeping the Ming capital and bureaucracy intact helped quickly stabilize the regime and sped up the conquest of the rest of the country. However, not all of Dorgon's policies were equally popular nor easily implemented.
Why did Dorgon keep Beijing as his capital? (If the question is unanswerable, say "unanswerable")
stabilize the regime and sped up the conquest of the rest of the country