In the Liturgy of the Hours, morning prayer is in the form of psalm/canticle/psalm; but, evening prayer takes the form of psalm/psalm/canticle. Is there a particular reason for this variation?
Depends on which form of the Liturgy of the Hours you use. The Benedictines order theirs according to their Rule, Franciscans order theirs to their Rule, the Church has set her own order. You might want to put this thread in Liturgy and Sacraments where you would get more help.