Probably many Daoist students still argue as of today about why Laozi has named the One with no name, Tao. Since Laozi had also mentioned in his Tao Te Ching that there is Tao of old, it may hold water if one makes the following suppositions.
In a few of my previous entries, one had assumed that the ancient sages including Laozi and Confucius have learned from the ancients before them, legends like FuXi, Yandi (Shen Nong) and Huangdi (Yellow Emperor).
When both Laozi and Confucius mentioned in passing the fall and taught the people how to return to Tao, they may have learned this from the ancients. Of the Classics of antiquity, perhaps both have learned it from the Zhou Book of Changes – the Zhouyi - said to have existed since the times of King Wen (c 1100 BC).
If the received text has not been adulterated, and remains the same as the original Zhouyi, we will find that the name, Tao, has been mentioned on two occasions - in the first line of Hexagram 9 Xiao Chu / The Taming Power of the Small and in the Judgment of Hexagram 24 Fu / Return.
The first line in Xiao Chu simply says: Return to the way. How could there be blame in this? Good fortune.
While the Judgment in Fu says: Return. Success. Going out and coming in without error. Friends come without blame.To and fro goes the way. On the seventh day comes return. It furthers one to have somewhere to go.
If you have not spotted the similarity of advice on both the line in Xiao Chu and the judgment in Fu, the Yi advise the Return to Tao (the Way). (In case you do not know why Daoists, Confucian and Buddhist cultivate, if there is no fall from Tao, why is there a need of a return? To return to Tao, we need to cultivate.)
On the assumption that the learned scholars during the Spring and Autumn era (c 500 B C) have access to the Zhouyi and other ancient classics, and with their intense investigations and pondering, perhaps both Laozi and Confucius understood Tao more than the others. Laozi went on to write down his thoughts on Tao and Te in the TTC, while Confucius taught his students the Middle Path. Both sages pointed to the Center for people to return to Tao.
If the suppositions are correct, then Laozi must have been influenced by what was written in the Zhouyi (similar to the many wise and learned down the ages) and therefore his reason to call the nameless One, Tao - just like what the ancients before him did.