Seems like it’s not just HD consoles that this game was considered before prior to being made for the 3DS, since Konami were at one point considering releasing the game on the Playstation Vita instead. Here’s the original quote confirming it:
The Vita could have run the game no problem, and we did seriously consider it as a [target] platform. But when we saw the game on 3DS and how it looked with the 3D, it made us realise we could never replicate that on another system.
Seems like the graphics and 3D effect had a pretty important role in getting this game made for the 3DS. But do you know what I think also probably had a minor effect?
The 3DS selling somewhat decently and Vita failing miserably. I mean, if the Playstation Vita and 3Ds had both sold about 10-20 million units at the time and had a similarly sized userbase, it would have probably be a multiplatform title to take advantage of this. But by the time Castlevania Lords of Shadow Mirror of Fate was officially announced and in development, the Vita had sold so few units that I suspect Konami considered the idea of releasing a major game on it as unprofitable.
Either way, it’s pretty much fantastic that us 3DS owners are getting this game this game this year, it looks like an excellent new installment in the Castlevania series and just the type of game the system needs to keep momentum going throughout early 2013 (alongside Luigi’s Mansion Dark Moon and some others).