Maybe that has something to do with the manufacturer's need to keep the hardware compatible with the software Microsoft had available. If some killer feature is a great idea but isn't supported by the operating system, what exactly is Dell or HP or Lenovo supposed to do? They built the devices the OS could handle, not the devices that would dethrone the iPad.
If this discussion were about poor response to, say, Google creating a hot new built-in-house Android tablet, you'd have a point. But unlike an open-source OS like Andrid, OEMs have never had the ability to change Windows itself, just the option of adding in bundled drivers and software. And as mentioned upthread, there was no incentive for Microsoft to work with one specific OEM to produce a feature that other OEMs wouldn't have.
If you want to blame the OEMs for anything, blame them for trying as hard as possible to make their devices look just like Apple's, while simultaneously (and largely unsuccessfully) trying to claim they were different and much better than Apple's devices. Almost every "ultrabook" mimics MacBook Air. There are lots of laptops that look like MacBooks. Nearly every slate/tablet (including Surface!) looks like an iPad clone, and many, many, many phones are as close to the iPhone design as they can get without getting sued. If they want a killer device they need it to look like it isn't copying Apple, because no one wants a knock-off.