Because dwc is the method most associated with root rot. Almost every root rot thread on riu is a dwc/rdwc thread. Your idea would be worse, because the contents of your rest would constantly run through a pump, raising temps in the root zone and inviting pathogens to multiply and decreasing dissolved oxygen. I thought about it quite a bit a few years ago and concluded it's too risky. I flood twice a day and get great growth rates. Flood and drain is the best. I think it's a true statement that roots thrive best when they aren't permanently submerged.