Here's why:
* Full rhymes have the same vowel sound and the same consonant sound after the vowel.
* "Witches" has the vowel sound of "itch"
* "Ditches" has the vowel sound of "itch"
* However, the consonant sound following the vowel is different: "ch" in "witches" and "ch" in "ditches".
Therefore, they are only near rhymes or slant rhymes.