They didn’t need to.
Do you honestly believe that after hundreds of millions of years of evolution our teeth evolved to always hurt and become crooked? Of course not.
This is a skull of an Ancient Roman:
An Anglo-Saxon princess:
An Ethiopian pre-Human:
As you can see, perfect, white, non-decayed, and non-crooked teeth. In fact almost 90% of all teeth of ancient and medieval people are perfect, a stark contrast to the 20th century, where only 4% of all teeth are considered naturally perfect.
The reason for this is quite simple. Today we rely on daily hygiene because of the amount of sugar and other harmful food that’s been in our diet since the discovery of the Americas and importation of sugar. The people of the medieval ages didn’t have a lot of access to sugar, therefore their teeth were virtually perfect.
Regardless, medieval people actually did clean their teeth, despite not needing to do so, mostly because they liked to smell nice. They used linen-cloths to rub their teeth and salt, rosemary, pepper, mint, powdered charcoal, and herbs to clean their teeth.