I wonder why the Algerians would refuse a chance to become Christian? Makes little sense to me.
The Algerians would probably say the same about you failing to convert to Islam. It seems that, as an evangelical religion based on a written scripture, Islam is quite sticky and is able to cultivate loyalty even in an environment where it is suppressed. Buddhism, Judaism and for that matter Christianity have pulled the same or a similar trick.
How were the Europeans able to convert lots of Africans further south, then?
Partly, they weren't; Senegal, Niger, Chad, etc. are deeply Muslim. Cote d'Ivoire, Benin, Congo-Brazza and Guinea Equateur had people who followed traditional religions which were largely based on ethnic affiliations and did not have written scripture, and these seem to be generally less immune than the religions I described above to conversion. Why, I don't know, and it would probably take a few volumes of books on religious theory to even begin discussing it, but it seems that as a meme these religions aren't quite as effective as propogating themselves. Even then though, traditional religions still have a lot of adherents, perhaps most famously with Benin's vodoun practices.
EDIT: Rather than reading volumes of textbooks, I guess you could read the post above mine.