Board Thread:Suggestions forum/@comment-26065088-20160320174315/@comment-25661985-20160321152637

Do you mean that the evil of Angmar were stained Forodwaith and formed its wasteland? However, it said: "The Forodwaith had been made very cold earlier in Arda's history because of the presence of Utumno and Melkor's evil cold emanating from it."

- Tolkien Gateway

You can find same description in LotR Appendix A, "The Númenorean Kings", "Eriador, Arnor, and the Heirs of Isildur". Basically, Forodwaith is cold because its pure evil, and that evil could not obe vershadowed by minor evil from Angmar.