But i think you should consider the following
1) Because it was original
Because Bionicle was not based off another brand, it allowed the creators to put some real imagination into this, you could say things such as Hero Factory are original as well, but Hero Factory is too similar to Bionicle and not as good.
2) Because it was cool at the time
In your pre-teen/teen ages now, you might not agree that LEGO is cool, but when Bionicle came out, it so was, kids my age would go out together to the shops and just blow out their money all on Bionicles because it made them look cool playing with them, and i'd have to agree.
3) The Video Game was good
I played Bionicle Heroes for Nintendo DS and to be honest with you, it was really good. It was a sort of shooting game where you were against the Piraka in different themed worlds (such as Ice and Lava), and it wasn't that easy but it was certainly fun.
Got any more reasons on why you think Bionicle is the best set ever or disagree with me, post below!