Its been hypothesized that the rosary is constructed as a substitute for the psalter that many religious recite.
There are of course 150 psalms, likewise 150 ave’s are traditionally prayed in the entire rosary. Although laypeople in the middle ages- and now, btw- don’t know the psalms and didn’t have them available to read- they all knew the Hail Mary. So it became a devotion that all of the faithful could do.
(the Luminous Mysteries are a new addition to the rosary of recent vintage, nowadays of course there are 200 Hail Mary’s in a complete rosary)
