Capcom is looking for sufficient demand
Outside of Japan, Phoenix Wright Ace Attorney - Dual Destinies will be available only as a download from the Nintendo 3DS eShop. That's the current plan, at any rate. Responding to fans, Capcom senior VP Christian Svensson explains that a physical release in the US isn't wholly out of the question. As these things tend to go, it'll come down to demand.
"So, if you'd like to be heard, you can post on this thread (I'll leave it open), PM me on Capcom-Unity or hit up my twitter (@chrissvensson) as many folks already have. I need some additional ammo and these requests fit that need," wrote Svensson.
"If things go well and demand is there, in a few weeks I may be able to present an option that puts the ball back in the fans' court (in "tennis" terms, not "of law" terms). I want to stress the word "may" so as to not misrepresent the situation or promise anything. We are trying. Again, timing and pricing would be at a disadvantage to the digital option as an assumption you should make."
A limited run would surely be appreciated by series fans. Would you hold out for a retail copy?
Eshiro (Producer of Ace Attorney) says a physical US release of AA5 is still possible (+AAI2 Tidbit) [Ask Capcom -- Thanks, AdamDuffield]