With an Apple iPad, your only decisions are: do you want a 3G modem, and do you want the super-resolution display? That's it.
With Windows, it's already much more complicated. Do you want ARM or Intel? What exactly are the tradeoffs? Performance? Battery life? Legacy compatibility? Which keyboard variant are you getting?
As to the Windows OEMs, I suspect that they're ambivalent about Microsoft's I-love-you-I-hate-you waffling. You engineer an ARM-based tablet. You get it running Android. You get it running Windows RT. You tweak the case a bit so you can give them different SKUs, and you offer both for sale and see what happens.
Since Android is "free" (although you need to hire your own devs to get it working on your hardware) and Windows costs money (and you may or may not need your own devs), it will be interesting to see whether Android devices end up costing less money. If that happens, Microsoft's in trouble, because for many consumers, there's Apple or Other, and Microsoft is lumped into the Other category. We know that Microsoft's solution to this problem is to try to place a patent tax on Android. We'll see how that shakes out.