why passé compose rather than plus-que-parfait

Just to show that the sentence you inquire about is NOT plus-que parfait, let's reword the sentence without changing the temporal relation:

You had your curtains changed. --> You asked [someone] to change your curtains.

In the second version it becomes clear that this sentence is simply imperfect tense in English, which becomes passé composé in French. The plus-que-parfait version would be:

You had had your curtains changed --> You had asked [someone] to change your curtains.

Rebecca, 

the use of ‘ had ‘ in English does not automatically invoke plus-que parfait in French. 

Plus-que parfait can be appropriate to describe a completed event before another later event, already known or being described. There is only one completed past action described here - so passé composé is used.

Have a look at ‘ La concordance des temps ‘ link :

 https://french.kwiziq.com/revision/glossary/verb-tense-usage/la-concordance-des-temps-sequence-of-tenses

Le Plus-Que-Parfait is used when describing past actions that OCCURED BEFORE OTHER PAST ACTIONS. In French, this rule is more put in place and emphasized. Because there was no action before, le passé composé or le passé simple (depending on the context) could be used.