When Latin evolved to French, the vowel /u/ fronted to become /y/... except in Latin "VRSVS" /ur.sus/ > French "ours" /uʁs/, in which the vowel /u/ was kept. I do not think that the /rs/ environment kept the /u/, because we can see Latin "DVRVS" /du.rus/ > Old French "durs" /dyrs/, where the "u"...