The short answer is that Sun Ce and Liu Biao were also fighting each other, they also had their own troubles to care about, some of which was incited by Cao Cao's machination. And in case of Sun Ce, he's dead.

First, let's talk about Liu Biao - who was Yuan Shao's ally. After the Guandong coalition campaign, Yuan Shu occupied a portion of Jing province. As an counter move, Biao was assigned to the other half of the Jing province by Dong Zhuo's court, his task was to dislodge Yuan Shu in Nan Yang - which he eventually succeed. Yuan Shu's vassal Sun Jian was killed and Shu himself had to fled east. In the following years, Biao came into conflict on 2 sides with Cao Cao in the north, Yuan Shu & Sun Ce in the east. While technically at war, the former two was both occupied with their own affair to put in any real pressure, Cao Cao did tried to march south but he was badly mangled by Zhang Xiu's surprise betrayal. Sun Ce, on other hand, sailed upstream to attack Biao relentlessly, no doubt partly motivated by his family vendetta. On the year 200, the battle of Guan Du started and Yuan Shao called his allies for aid, Biao did not act. The Record of three kingdom attributed this to Liu Biao's own indecisiveness and pettiness. It could be true that Biao did not want to take actions because of personal flaw. Or because he was wise enough to know that he couldn't.
At the very beginning of that year, Sun Ce destroyed Biao's ally Liu Xun and also defeated Biao's general Huang Zu. Despite still having a considerable forces, Liu Biao had to brace himself for another invasion from Sun Ce very soon. As far as the governor of Jing knew, this Sun Ce seemed to be 1.invincible so far, 2.wouldn't settle for anything less than Biao's blood (plus his family's, his friend's and his horse's) and 3.was Cao Cao allies. He would be begging for Sun Ce to act, should he diverted his forces to the north.
In an even more dramatic event, Zhang Xiu - who has been Liu Biao's ally against Cao Cao - switched side and defect to Cao Cao. This timing move should curb any further thought of marching north for Liu Biao.

Sun Ce was a much more simple case. Ce was a loose ally of Cao Cao but the relationship was uneasy at best. Cao Cao appeased him with title and marriage offers, sanction Ce's war with Yuan Shu and Liu Biao, hoping Ce can keep these 2 in check. By 200, Yuan Shu is all but gone and Liu Biao has been put in a defensive position; basically, Sun Ce now had freedom to do whatever he pleased. He can continue his push against Liu Biao, he can turn around to stabilize his domestic affair or he can take the opportunity to take his pieces in the Guan Du's conflict. The Record of three kingdoms wrote that Ce did intended to backstab Cao by attacking the capital city of Hua. However, in summer 200 when the fighting at Guan Du started, Ce was busy putting down rebellion by Yan Bai Hu and Xu Gong. He was assassinated in May. Whatever he intended to do next was put to a stop.

Sources:
De Crespigny, Imperial Warlord, chapter 3
De Crespigny, General of the South, chapter 3
Record of three kingdoms, Liu Biao biography chapter & Sun Ce biography chapter.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

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