Personally, I think Dong Zhuo didn't reuse Liu Bei for the following three reasons. According to the related contents in the romance novels, the analysis is as follows.
First, Dong Zhuo despises Liu's "white body" status.
At that time, Liu Bei saw the imperial list and recruited soldiers to destroy the Yellow Scarf Army. When he was feeling deeply, he met Zhang Fei and Guan Yu successively. They shared the same interests, so they became half-brothers in Taoyuan and joined the army. After Liu became sworn, they raised funds to build weapons and recruit soldiers. At this time, Liu Bei was not appointed by the Eastern Han court, but was naturally a "white man". So after he saved Dong Zhuo, he was asked "Where does he live now?". Liu Bei was embarrassed to lie, so he could only answer honestly: "White body". "White body" is ordinary people. No official position, just an ordinary person.
Liu Bei's answer is also very helpless. Although he is a relative of the Han family, his generation can only make a living by selling straw sandals because of the "favor order". Whenever Liu Bei is short of money, he is better than the white man, but he is not. At this time, Liu Bei didn't know what it was like. It is estimated that there are ups and downs. Later, in Surabaya Pass, Guan Yu was also despised by the leader Yuan Shao, who thought it was shameful to send "horse archers" to fight Hua Xiong.
Secondly, Dong Zhuo may know that Liu Bei is a student of Lu Zhi, so naturally he dare not attract reuse.
In the fourth year of Xiping (AD 175), when Liu Bei was fifteen years old, he joined Liu Deran, Gongsun Zan and others to learn from Lu Zhi, a native of the same county. Liu Bei, a student of Lu Zhi, is going to Lu Zhi to be in charge of encirclement and suppression of the Yellow Scarf Army. However, on the way, Liu Bei heard that Lu Zhi was "ineffective in encirclement and suppression" and was dismissed by the court and replaced by Xiliang Dong Zhuo. The reflection records are as follows:
15 years, my mother made a trip to school. Both Liu Deran and Gongsun Zan of the same clan had an accident, and they were in the same county as Jiujiang prefect Lu Zhi.
Dong Zhuo is the new corps commander. He may find that Liu Bei was a student of his predecessor, Lu Zhi, and he came to join Lu Zhi, but he replaced Lu Zhi. Although this was the decision of the court, I took the place of Lu Zhi after all. In a unit, the current leader generally does not reuse the cronies of the former leader. Everyone has his own team, mainly his own small group, which is handy. Obviously, Liu Bei belongs to Lu Zhi, and Dong Zhuo doesn't need it at all, considering that it is difficult to take it for his own use. If Lu Zhi is saved by Liu Bei, everyone will be happy, and most of them will be reused and made contributions.
Third, even if Dong Zhuo recruited Liu Bei, he would not be entrusted with an important task.
Liu Bei's saving Dong Zhuo was a personal kindness to Dong Zhuo, and Dong Zhuo appointed generals as state officials. Public and private can't be confused. Dong Zhuo can't appoint Liu Bei and others because Liu Bei saved his life. At this point, Dong Zhuo has done a good job and deserves praise. There are thousands of ways to repay kindness, such as giving away gold and silver jewelry and recommending it to the court. In short, private interests should not be wasted because of public interests.
Even if Dong Zhuo recruited Liu Bei, he would not be entrusted with an important task. For example, Zhao Zengyun saved Gongsun Zan and defected to Gongsun Zan. Zhao Yun saved Gongsun Zan's life, and his martial arts is good. As soon as he made his debut, he was able to draw with Wen Chou, the general of Hebei Province. In emotion to reason, Gongsun Zan should also reuse zhaoyun. But Gongsun Zan only used Zhao Yun as an ordinary general, so he was very wary of him and didn't make good use of it. After Zhao Yun was surrendered by Gongsun Zan, he found a reason to go.
If Dong Zhuo recruited Liu Bei, most of them would end up like this. Dong Zhuo dared not give Liu Bei too much military power. If you are bitten by Liu Bei, you will lose more than you gain. Dong Zhuo's worries are not unreasonable. For example, Dong Zhuo trusted Lu Bu very much and accepted him as his "adopted son". He will think of him as soon as he has good news. I didn't expect Lu Bu to slay Dong Zhuo with his backhand.
Summary: It is precisely because Dong Zhuo has the above three concerns that he did not recruit and reuse Liu who saved his life. Although Guan Yu and Zhang Fei were skilled in martial arts, they were not famous at that time, and Dong Zhuo didn't know that they were "enemies of ten thousand people". If Dong Zhuo knew that Liu Sanying in Hulao Pass was fighting against Lu Bu, he would probably regret why he didn't recruit these three great heroes.