One of the big reasons is ductility. We want structures to show signs of distress before failing catastrophically. With ductile reinforcing (steel), the deforma

One of the big reasons is ductility. We want structures to show signs of distress before failing catastrophically. With ductile reinforcing (steel), the deformation at which failure occurs is easily 5 times larger than the deformation where yielding occurs. Large cracks will open up, it hopefully gets noticed, and there is a chance to intervene. With GF or CF reinforcing, failure is sudden and catastrophic.