Not licensed?
Sarcasm aside, Matt’s nod to only licensed sets making it through LEGO IDEAS is something to consider. For sets to be successfully adopted, LEGO’s marketing branch has to determine that there will be a demand in the market place. The voting only gets their attention long enough to consider the proposed ideas.
It seems to me that if you want to get an idea approved, you have to consider the marketing angle. I am no business major, but I am sure that TLG knows that people are most likely to spend money on something
1) with which they identify strongly or which evokes an emotional response
2) contains parts / figs they want but cannot get elsewhere or
3) fills a perceived need.
Of course none of this explains failed LEGO product lines like ZNAP or how dollar stores stay in business, so I realize my hypothesis, and grammar, are wide open to criticism.