Being more centrally located in terms of industrialization (Manchuria), having been the original and non-Chiang capital of the ROC, geopolitically safer (Shandong and Liaoning being a good barrier), closer to the USSR, less damaged than Nanjing, and originally the most ideologically red of the historical Chinese capitals (Beijing University), with the added honor of where the May Fourth Movement started.
I think these would be the main reasons.