apart from what believers claims....
I think the main reason is not because of what they bible teaches, but rather what it leaves out.
the other myths of mankind over explain everything to the point of absurdity in later generations....
but the bible is more like the old bugs bunny cartoons.... they were not made for idiots or children but for adults
and as you grow older you see more and more things you had no clue about as a child....
the bible also has a lot of oddities tossed into the mix that let the mind run wild.... like where did Cain get his wife?
what was Jesus doing from age 12 to 30?
The trinity is popular because the bible never actually spells it out and and gives enough odd sayings and near contradiction to allow
people to read it backwards into the text....leading to debates which began from nearly the beginning of christianity.