Because, "Shu(蜀) orthodox theory" is claimed by the time of the east Jin(東晋) dynasty(317-420), and novels(including romance of three kingdoms) are also affected.
And it is largely due to book（Cáo mán chuán 曹瞞伝） episode that was enemy Wu(呉) human wrote the novel is adopted. And those episodes so interesting regardless of the truth, it was supported by the masses.

Moss Roberts, who did a very good translation of the unabridged Three Kingdoms, talks extensively about this in an afterword, as well as reasons why Zhuge Liang was so built up, later editors' revisions and commentaries on the themes in the book, very interesting stuff.

So Luo Guanzhong obviously wasn't going off the best record of the times, The Chronicle of the Three Kingdoms by Chen Shou, because Chen had to uphold Cao-Wei's legitimacy to lend Jin credence, so Cao Cao is hailed as the "Grand Progenitor" and given the highest esteem by far. The reason doesn't lie in the historical record, but the time that Sanguo yanyi was written, which scholars believe was probably the late Yuan period, and possibly the early Ming, in the 14th.

After the Three Kingdoms, when Jin fell apart after the War of the Eight Princes, China split into many kingdoms, which typically named themselves as new versions of states from the Warring States period, before the Qin dynasty. The states in roughly the northern half of China during the "Southern and Northern and Dynasties" era were ruled by Xianbei, as in Wei, and "sinicized barbarians," as in Northern Qi.

This repeated during the Song dynasty, with the Liao Tatars, Jurchens, and finally Mongols ruling the former north of Han China, eventually conquering the Southern Song as well. As Ming resistance was likely coinciding with Three Kingdoms being written, several scholars such as Qiu Zhensheng wrote that Luo Guanzhong was motivated by anti-Yuan (Mongol) Han nationalism. Qiu wrote

...a pro-Liu, anti-Cao view...had been taking shape since the Song and Yuan. For Luo to have affirmed Cao Cao would have been tantamount to affirming the legitimacy of the Yuan dynasty which occupied the northern heartland and extinguished the Song dynasty. Luo Guanzhong, who 'strove for kingly government,' could never have accepted this...and he poured his unrealized ideals into the figures of Liu Bei and Kongming, the sagely sovereign and the able minister. (Qiu Zhensheng, Sanguo yanyi xingcheng gu ocheng lunlue, p. 68.)

Since the Three Kingdoms stories held such cultural sway long before, and independent of, Romance of the Three Kingdoms, it was useful to have Liu Bei live on in Zhu Yuanzhang, leader of what would become the Ming dynasty, fighting against a northern tyrant, Cao Cao, from the same place Liu Bang began the Han dynasty, in modern day Sichuan. There are other theories, but I find this the most persuasive.