What Diz said. A tablet can't run the same games a PC can (yet), they need to be severely optimized in terms of polycount, texture sizes, etc. (Yes, even awesome-looking games like Infinity Blade are probably way more optimized than one would think.
) That's also the reason why you can't find any major games in the store - if it was that easy to convert an existing PC or console game for tablets, the big developers would've already done it!
Also, not every engine is compatible with tablets and their operating systems (not 100% sure if this would also be the case for a tablet that runs on windows...well even if the OS works, there might still be hardware-specific issues I guess). Some engines make it fairly easy for developers to develop for multiple platforms, like Unity or Unreal, but Frictional coded their own engine which therefore does not have this feature.
So in a nutshell, it would be a tremendous amount of work for them (if not impossible to do without re-coding half the engine and redoing all the assets) with very little money in return. Because not only are apps pretty cheap compared to PC games, there are also not that many people who own a tablet and are also interested in a game like Amnesia, compared to the big fanbase on PC.
And last but not least, I simply can't imagine a game like Amnesia working on a tablet in terms of immersion and atmosphere... but maybe that's just me.