imo there's 3 reasons:
Ignorance
Many ppl don't know anything about metal, since normal hardly hear anything about it, except perhaps, the negative things, or don't know where to get it... of course, when you already inside, there's no problem at all.
Social
social background...
Many ppl are afraid of going their own way. If all your friends listen to techno it's harder to give real metal a try, except for nu-metal since it's played everywhere nowadays.. and of course, ppl are afraid of being regarded as retards by the ppl they socialize with (who's not into metal).
a funny quote by my sister: " what's the point in listening to music that nobody else listens to?", meaning that many ppl just simply want to listen to the same music as their friends etc, if you follow me. It's more fun to share your interest in music than enjoying something on your own... else ppl wouldn't be on MR anyway.
and Metal aint trendy "enough". If you only care about walking around in expensive clothes you don't care about metal either. of course there are trends within metal, but they don't matter in the daily life of normal ppl... no no... Paris matter in that context etc.
Music - Taste, Adaption and interpretation
1: there's the taste in music - and many is satisfied just by listening to pop in the radio, since it's sooo easy.. no work at all.
2: Metal also requires the ear to adapt to the new form of music. Some types of metal are harder to get into, when you're new. So naturally, you wont start listening to death and black right away, unless you know ppl who already listen to those genre.
3: Metal requires bla bla... i for one would use the term "complex", but only in the sense that most metal aint build up like popsongs, with "A A B A B A B", and you can't party to metal the same way you can with techno

or sing along to catchy choruses, since alot of chorus aint catchy - many choruses do become "catchy" when you have learned to enjoy metal though.
something like that...
