Like they did with BT1, they added different tests for the English version to replace the Japanese oriented stuff. I think that they may be having trouble finding more tests to replace the Japanese stuff.
Personally, I thought that the BT1, English version was better than Japanese version because it had slightly better handwriting recognition (useful for shaving those extra seconds off the math 20 and 100)
and imho it had the best implementation of Sudoku on the DS (or any electronic device), I just wish it had more boards.
Also, based on history, Japan seems to reward franchises more than original titles (as long as it's not "terrible"). Sequels of good movies often do better, similarly with games. So this could explain the success of BT2 regardless of whether it was "better" or not
