Guan Yu is not revered simply because of the Romance of the Three Kingdoms.

‌It is not because the Romance of the Three Kingdoms became widely popular that he was revered.
He was originally worshipped as a local indigenous deity—and many other characters from the Romance of the Three Kingdoms were like this as well.
During the Song Dynasty, salt merchants accumulated enormous wealth through the salt monopoly system,
and Guan Yu was the local deity of Shanxi Province, the central salt-producing region.
Since credibility is crucial in the salt trade, they began to revere Guan Yu, a local deity known for his loyalty and righteousness.
By turning this into a formal religion through the use of religious statues (like the small idols you see in dramas or movies),
his deification spread nationwide...