1. It's Vita. Sales are dire in the UK and it has a tiny presence in retail stores which is vanishing fast.
2. Next to no advertisement.
3. It released around the same time as PS4/XB1 and probably got dwarfed by the hype and mindshare for them.
This is sad. A great developer makes a great game and it sells sweet FA, yet assholes can make ass-y games and sell millions and millions. Man this stuff depresses me.