Why is this a requiem?
A requiem is defined generally as a musical composition for the dead. This can include a full orchestra (like Mozart's Requiem) or it can be as simple as a chant to commiserate the passing of a person.
This word, requiem can represent a spiritual death namely the death of spirit of colored girls and of the different phases of life that a 'colored' girl goes through before she finds herself. In the last section, there is a rebirth of sorts as the women realise that everything they need is inside themselves and not externally.
The play is actually a set of poems set to music - so there you have the reinforcement of musical composition. Where it differs from a traditional requiem is that at the end of the play, a new woman emerges who has found herself. In a traditional requiem one is lamenting the death of either an important person or a loved one. Here the audience would understand that this death is positive as it leads to the birth a new person. The last line of the poem repeats the first line of the poem except that it is the present tense - "& this is for colored girls who have considered/suicide/but are movin to the ends of their own/rainbows."