why was Henry VII called the "son of prophecy" or "mab darogan"?

The short answer is Tudor propaganda.

When discussing Henry VII, it's important to remember that his dynastic claim to the throne of England was incredibly weak. His mother, Margaret Beaufort, was the attainted descendant of the fourth son of Edward III. His father, Edmund Tudor, was also the half-brother of Henry VI through their shared mother, Catherine of Valois. While the Yorkist claim to the throne also passed through the female line (Richard, Duke of York's mother was a descendant of Edward III's second son), their claim preceded that of Henry Tudor and they were also full-blooded Plantagenets through the male line. By contrast, the Tudors were a Welsh family, giving them a distinct outsider status in England. From a purely dynastic standpoint, it would have been hard to argue that Henry was anything other than a usurper of the throne of England.

Moreover, Henry Tudor's political position was also weak. When Henry crossed the Channel in 1485, the Lancastrian faction to which he belonged had essentially already lost. Henry VI mysteriously died in 1471 and all of the other male-line members of the House of Lancaster were either already dead or had been killed in battle. Henry Tudor became Henry VII not because he was the best candidate, but largely because he was the only real alternative to Richard III left. His primary supporters were the rebellious Yorkists and crypto-Lancastrians that opposed Richard as well as the king of France. To many Englishmen, Henry would likely have been an upstart Welshman leading an army of traitors and even worse, the French.

As you can imagine, this dynastic weakness left Henry VII (and his descendants) in a particularly precarious position. One of the ways that Henry and his descendants dealt with this insecurity was through the active production of propaganda. This is why Shakespeare's Richard III, produced during the reign of Henry VII's granddaughter, portrays the titular king as a grotesque scheming monster.

This is where the mab darogan and the prophecies enter the picture. One of the many avenues of Tudor propaganda was the co-option of Welsh prophecy and myth. Wales had a long tradition of prophetic poetry, darogan, which was closely tied to Welsh identity and politics. One of the central figures of this tradition was the mab darogan or "son of prophecy," a hero who would one day free Wales from Saxon oppression. As the scion of a Welsh family, Henry actively cultivated the idea that he might be the mab darogan arriving at last to free the isles from an English tyrant. He landed his army in Pembrooke and fought Richard III with the red dragon of Wales on his standard. His first son and intended heir was named Arthur, the name of one of the most famous possible incarnations of mab darogan.

The casting of Henry as the messianic hero can be seen in the propagandistic prophecies produced under his rule. Proffwydoliaeth y Wennol/Prophecy of the Swallow, a fanfic of sorts to the existing prophecy of Ceiliog y North/Cock in the North, describes a swallow (clearly intended to be Henry) returning to Britain to vanquish the enemies of the lion (England) and dragon (Wales) described in its predecessor. In short, the Tudors argued that along with being dynastic heirs to the throne, their triumph over Richard III was destined to happen.

Sources:

Fisher, Deborah. Royal Wales. Cardiff: University of Wales Press, 2010.

Flood, Victoria. Prophecy, Politics and Place in Medieval England: From Geoffrey of Monmouth to Thomas of Erceldoune. Woodbridge, Suffolk: Boydell & Brewer, 2016.

Hicks, Michael. The Wars of the Roses. New Haven: Yale University Press, 2010.

Jones, Aled Llion. Darogan: Prophecy, Lament and Absent Heroes in Medieval Welsh Literature. Cardiff: University of Wales Press, 2013.