The obvious reason is that hardware requires a business model for distribution. Hardware also costs money to make. Why invest in what everyone else can make as well?
That's why Open Hardware talks about bringing the “open source feeling" to hardware, not open source designs. And why open source designs never seem to make it.
There's a difference here between open standards and open source. Microsoft's is an open standard, which spurred a host of manufacturers to produce PCs. WiFi is an open standard, and there are many places to get wireless routers.
So why, if open standards work in the hardware market, doesn't open source?
I'm thinking it's the lack of a powerful sponsor. A vendor with a marketing budget, or an industry with a shared one, is needed to get hardware over the hump.
Is that all open source hardware needs, then. A co-op plan?