Why are rift zones common places for igneous rock to form

They are commonplaces because they have deep cracks where magma can come from. And we all know that igneous rock is molten lava that has been cooled, so this answer will make sense.

Rift zone is the area on the Earth in which plates of the earth's crust are moving away from each other, forming a zone of fissures (gaps). From that zone of fissures, lava (or magma) erupts. That lava when cooled down makes the igneous rock on the surface of the Earth. That is the reason why the rift zones are the common places for the igneous rocks to form. 

Wish i knew this to !