Xxx.3gp Why Can't I Import Movies "xxx.3GP" Into IMovie?

Why can't I import Movies "xxx.3GP" into iMovie? - xxx.3gp

I even tried the 3 "GP" to "zip". The works of cinema, but I can not import. Work only in "xxx.MOV" or "xxx.avi" seems.
Why? What can I do? How can I import them?

Thx!