It's about how many other people own both.
You left out GameStick; Mojo by mad catz; Nvidia Shield; GamePop; A range of tablet consoles from JDX; Archos Gamepad
Assuming that by JDX you mean JXD, maker of the S5110 gaming tablet: What are the sales figures for these product lines? I'm occupied this weekend, but I'll make a note to ask for these brands in both Best Buy stores in my area this week and get back to you. But I suspect the answer will be "look for them online", which doesn't help gamers become aware of these devices' existence in the first place. Something not sold in stores isn't likely to build up an installed base big enough to convince the big players that a port is worth the investment. It's the same reason that few to no commercial games were made for the GP32, GP2X, Wiz, and Caanoo.
and this is before heavyweights Amazon and Google both rumoured to step into the ring.
Anything official yet, as of late August 2013?
That is only for games "designed" or Playstation Controller first, those designed for screen input first will obviously have even less of a problem.
How would you recommend designing a Mega Man or Castlevania style platformer or a Street Fighter or Mortal Kombat style fighting game "for screen input first"? I tried playing Tetris on my aunt's iPhone and I couldn't see how one could achieve anywhere near the TPM that one can get on, say, Tetris DS. Then I tried playing the demo of Pixeline and the Jungle Treasure on my first generation Nexus 7 tablet, and the controls were so uncomfortable and gave so little feedback as to the position of my thumbs that I wished I had an NES controller to phone adapter. I couldn't even make a jump near the beginning of the second (or third?) level because of this. Platformers in Nesoid had similar problems. So ultimately, as I see it, the input method for which a game is designed first should depend on the genre. But well-known devices that ship with a gamepad have historically had far higher barriers to entry. In fact, some people have told me that it's actually the other way: the genre should depend on the size of the company, and indie developers should keep their ideas for games in gamepad-centered genres to themselves and make games in point-and-click genres until they're big enough to qualify for a Sony or Nintendo license. But I'm open to evidence otherwise.