Christianity came along and basically started trying to cast all religions within the same bounds that they used (Father, Son, Holy Ghost, Evil and Good, Heaven and Hell)
Other prominent examples include Hercules becoming the Son, Zeus being the Father, and various characters (usually Hermes) representing the Holy Ghost.
Other religions include Norse Mythology, Loki is not evil, he's a devious trickster, Odin is not exactly good, Thor is not the son figure, etc etc.
And of course Egyptian mythos, Ra is the Father, Horus is the Son, Seth is evil, Osiris sometimes gets cast as the holy ghost. Basically it's all down to the imposition of Christian values onto other cultures as they expanded. Nowadays it's just an accepted cultural norm.