Why do they build new factorys on green land?

within 10 miles of where we live there are loads of empty factory's. but instead of redeveloping them they are abandoned and new factory's are been built on an old moor land. when they are built they sit empty for a year with a for rent sign on the side. why do developers do this? the factory's clearly aren't need as the sit empty. should they be forced to redevelop the old sites, instead of building new ones?