Because in harry potter mystic numerology is real and Arithmancy is taught at hogwarts. And seven in christian numerology is the number of perfection and completion, taken from the seven days of the week which are in turned tied to god's creation of the universe in genesis. And since Advra Kedavra is a functional curse in Harry Potter that originates from the same tradition...
Other cultures and religious give seven powerful connotations for both mathematical reasons and being the number of days of the week in most calendars. Which results in most forms of numerology giving it special properties.
Wizard culture took the idea from the Latin/Christian culture. Wizard culture is nominally cristian as shown by the grave in Goodrich's Hollow, Christians believed that seven was an important magical number (seven heavens, seven deadly sins, seven sea), and Harry Potter wizards believe in numerology. And therefor so did Voldemort. He wouldn't pick five or six as they're numbers his tradition associates with man and therefor mortality, which in turn rules out ten. The other powerful numbers are either three or four which would give him less horocrux, or he could go all the way to twelve which has similar connotations to seven.
In short JK Rowling picked seven being magically important out of real world numerology, and thus didn't feel the need to explain real world mathematics in an already very long book.