why does the poet use ancient, stoppeth and wherefore instead of old, stops and why respectively?

The poem is written in archaic language, making it unique as far as Colleridge's poems go................ the use of words; stoppeth, stopp'st is a form a anaphora.

“The Rime of the Ancient Mariner” is written in loose, short ballad stanzas usually either four or six lines long but, occasionally, as many as nine lines long. The meter is also somewhat loose, but odd lines are generally tetrameter, while even lines are generally trimeter. (There are exceptions: In a five-line stanza, for instance, lines one, three, and four are likely to have four accented syllables—tetrameter—while lines two and five have three accented syllables.) The rhymes generally alternate in an ABAB or ABABAB scheme, though again there are many exceptions; the nine-line stanza in Part III, for instance, rhymes AABCCBDDB. Many stanzas include couplets in this way—five-line stanzas, for example, are rhymed ABCCB, often with an internal rhyme in the first line, or ABAAB, without the internal rhyme.

I didn't really know if you were looking for literary technique with this question or just a simple analysis. Therefore, I've added some information from other places. Note that anaphora can take on a few different forms, and that in addition to being wriiten in archaic language, the author wrote this poem as a lyrical ballad.

"In the poem's first line, we meet its protagonist, "an ancient Mariner." He stops one of three people on their way to a wedding celebration. The leader of the group, the Wedding Guest, tries to resist being stopped by the strange old man with the "long grey beard and glittering eye." He explains that he is on his way to enjoy the wedding merriment; he is the closest living relative to the groom, and the festivities have already begun. Still, the Ancient Mariner takes his hand and begins his story. The Wedding Guest has no choice but to sit down on a rock to listen."