Why do so many people think Nigeria is Muslim..?

At the moment, there's not a clear divide between the two groups. It's pretty much 50-50. Some estimates have made it much higher in the favour of Islam and others give the edge slightly to Christianity. So saying Nigeria is Muslim isn't really ridiculous and with the focus on Boko Haram, It's going to be overestimated some more.

But just being Nigerian doesn't give any authority on the religious demographics of the country anyway. Especially when a lot of British Nigerians are from the south and their parents likely Christian.

It is ridiculous to say it's a Muslim country. Just because you've seen people fleeing from an Islamist extremist group in one part of northern Nigeria, you start to think that the entire country must be Islam? That is absolutely ridiculous.

My argument isn't that Nigeria is Christian or Muslim. I'm not on some sort of mission to represent the Christians of Nigeria. I wouldn't even necessarily call myself Christian. I'm saying some don't seem to realise that its a country of mixed religions, and rather seem to think its a fully Muslim country due to recent coverage on Boko Haram. I was wondering what others thought of this and how it could have come about (perhaps other than from coverage on Boko Haram).

And being Nigerian most certainly gives me enough authority and knowledge to tell you that there are many practiced religions in Nigeria, and no one religion can necessarily be seen as the majority.