In one word: luck.

However, there are two important further points:

1. That luck needed to be backed up by a powerful military, an economy that could support that military, the administrative and political skills needed to build that military, and the political and military skills to put that military to effective use.

2. The Jin conquest of northern China also benefited from luck, and the factors in point 1 above.

Thus, what was the difference? Let us begin with the Jin.

The Song-Jin wars of 1123-1127 caught the Song when they were significantly weaker than usual. Their wars earlier in the century against Xi Xia (Tanguts) had been expensive, and the latter part of those wars had been unsuccessful, and had resulted in heavy losses. Partly as a result of the economic damage from those wars, the Song faced multiple major revolts in 1119-1121, losing control of large parts of their territory for months. These revolts were expensive to suppress, diverted military forces away from where they could fight the Jin, and killed millions. Despite this, the Song proceeded into more war, against the remains of the Liao (Khitan) state. This war was in alliance with the Jin, but the Song made little progress, and gained little territory. It cost them more money, and weakened their armies further. Again, despite this, the Song basically pushed the Jin into war against them.

• This weakness of the Song at the time was one major piece of luck for the Jin.

Four years of warfare against the Jin resulted in the destruction of the Song field armies, and Kaifeng (their capital) was captured. The Song still held most of their territory in the north. However, along with Kaifeng, the Jin had captured the Song emperor (and his just-abdicated father, the former emperor). The Song state had been decapitated! But the two former emperors were still alive (but in Jin hands). The Song, of course, promptly had a new emperor, the 3rd in rapid succession. However, their field armies were still destroyed and time was needed to rebuild them. Further, if the Jin released the former emperors in their captivity, the situation would have been most delicate politically. Rather than continuing to fight actively in the north, the Song rebuilt.

• The capture of Kaifeng, with the capture of the emperor and former emperor, and major disruption of the Song government and administrative apparatus was a second major piece of luck for the Jin.

After rebuilding, the Song continued to fight the Jin, neither side making any great progress. Both states were powerful - the Jin much more so than before now that they had the resources of northern China, but the Song still had plenty of resources. Thus, the almost hundred year stalemate up to the Mongol-Jin wars.

• The Mongol attacks on them were a major piece of bad luck for the Jin.

The above, and successful Song resistance against the Jin after rebuilding in the south was discussed by myself and u/lordtiandao in https://www.reddit.com/r/AskHistorians/comments/c7x73k/why_was_song_china_so_culturally_and_economically/ and myself and u/TechnicallyActually and u/lordtiandao in https://www.reddit.com/r/AskHistorians/comments/93nr8a/why_was_song_china_struggling_to_resist_the_jin/

Next, the Qing. The Manchus had already spent two and half decades grinding into Ming territory north of the Wall by 1644. This had given them more resources, both economic and military. The Chinese Banners brought not only more soldiers into the Qing armies, but soldiers skilled in the use of modern gunpowder weapons. Still, even with this, the conquest of territory south of the Wall would not be easy, even if possible.

• The opportunity to form the Chinese Banners and have a stronger artillery and firearm component in their army before the conquest of Ming China was a piece of luck for the Qing.

The Ming had similar problems to those the Song had immediately before their wars with the Jin. However, the Ming's problems were worse. Economic collapse, drought and famine, and major rebellions. Their problems were so much worse that one of these rebellions toppled the Ming. Li Zicheng's rebel army, large and well-equipped with artillery, captured Beijing. The Ming emperor committed suicide. Li Zicheng proclaimed a new dynasty. The Ming state had been decapitated, and not by the Qing.

• The fall of the Ming was a really major piece of luck for the Qing!

Enter Wu Sangui, the general in command of the most powerful remaining Ming army in the north, who had been on his way (with his army) to defend Beijing but had been unable to get there in time. He wasn't the only one - of two other who didn't make it, Tang Tong surrendered to the rebels, and Huang Degong stood back with his army to see what would happen - he wasn't strong enough to defeat the rebels on his own. Neither was Wu Sangui, so he did the same. His army was in a unique position - on the Great Wall, between the Qing army and the rebel army in Beijing. He had been protecting China north of the Wall against the Qing, and had abandoned that territory in his attempt to defend Beijing.

• The abandonment of the region north of the Wall, allowing the Qing to move in easily, was a piece of luck for the Qing.

Wu Sangui opened negotiations with both (the Qing also negotiated with the rebels). The Qing wanted submission, not cooperation as equals, so Wu Sangui wasn't (yet) willing to work with them. The rebels were looting Beijing, raping, torturing people to discover hidden wealth, etc., which didn't make Wu Sangui willing to work with them either. Li Zicheng wasn't happy with the lack of immediate surrender, and attacked Wu Sangui. The first two attacks were readily repulsed, which made Li Zicheng unhappy. The result of this unhappiness was Li Zicheng leading an army to attack Wu Sangui that greatly outnumbered his, which pushed Wu Sangui into cooperation with the Qing to avoid being defeated by the rebels. This made Li Zicheng even more unhappy, leading to the execution of Wu Sangui's father (who had been in Beijing when it fell) and other members of his family. Wu Sangui submitted to the Qing, their armies combined, and they defeated the rebel army, and liberated Beijing.

• Wu Sangui's defection was a major piece of luck for the Qing. Had Li Zicheng and his rebels in Beijing behaved better and negotiated better, perhaps Wu Sangui would have fought on their side.

This gave the Qing control of Beijing, and the start of Ming forces joining them. Other Ming generals followed Wu Sangui's example and joined the Qing. These Chinese forces were important, perhaps essential, over the next two decades as the Qing fought to control the rest of China. The key Ming generals, Wu Sangui among them, were rewarded with large fiefs and significant autonomy in the south (later revolting - the Three Feudatories Revolt - when the Qing took steps to prevent these fiefs from becoming almost-independent hereditary kingdoms).

So, in summary, both the Jin and the Qing benefited from luck. The Qing had more luck.