Win Phone 8 and Win 8 are right now two different Operating Systems.
They are worked upon by different teams AFAIK. So, I don't think the same strategy applies for both. A preview for Win Phone, if buggy, could screw up or even Brick devices. For Win 8 we can use recovery or work on partitions to get back but for Win Phone 8 once a device is bricked it is tough to recover it.
This might be the reason, but a preview/beta for developers should be outed, just like iOS.