why we consider the poems the rime of the ancient mariner for coleridge as romantic poem ?

Romance in this sense means the time and style it was written in. The Romance period started in the mid to the late 1700's. The "romantics" wrote about themes involving “heroic isolation of the artist or narrator, and respect for a new, wilder, untrammelled and "pure" nature." If we consider Rime of the Ancient Mariner we can see this. The narrator feels isolated, especially after the killing of the albatross. The power of nature and the spiritual world resonates throughout the poem (storms, ghosts....) This poem is a good example of the romantic period of literature.