I guess many choose LC:TBG because it connects to LC, even if with an inferior graphic design (the graphic design for either LC or Keltis is significantly better than for LC:TBG). I also think the rules for Keltis work better.
But LC is a very popular game here at BGG so that makes LC:TBG more popular than Keltis.
You talk as if the two games are different? I was told by the OP that these are the same game just different "versions".