A friend of mine is doing a research paper on this. There's a couple of reasons:
a) they are just sexist pigs
b) some men believe that they cannot call themselves feminists and instead something like allies
c) feminist is a loaded word and many women, even, associate it with the bra burner who hates men
d) others adopt the term "womanist" as opposed to feminist since they think womanist is more inclusive of WOC
Just my two cents anyway
EDIT: I would call myself a feminist since I don't think a/b/c are fair or reasonable. I understand WHY someone would call themselves a womanist but
I believe that feminism just needs to be pushed to be inclusive of WOC