[aaaaa]

Is the game genre of KB:L -- turn-based tactical RPG -- to blame? I don't think so.
There are lots of popular turn-based (tactical) RPG, especially Japanese RPG like Final fantasy (tactics), Super-Robot War, Front Mission, and Sakura Taisen series on game console, Jagged Alliance, Fallout, Oblivion series on PC, to name a few.
Turn-based battle on hexa-grid is standard battle scheme on most RPG games., be it JRPG or Western RPG, on console or PC. There are just too numerous popular RPG game with turn-based battle to list here. Many of them are mega-selling, hot blockbuster title even in recent years.

So turn-based battle system is the least probable cause of low sales number.
On the contrary, I think that tactical battle system of KB:L (and HOMM in this regard) has strong edge over compared to that of other hot Japanese PRG's on console with similar turn-based battle system.