I'm using Islamist as the term was first defined, which means exactly "Islamic terrorism". As far as I know there's never been a popularly-agreed-on meaning of "Islamist" other than that, if you're referring to the peaceful followers of Allah the term is Muslim, or perhaps Islamic, but never Islamist.
Christian on the other hand is already overloaded with many meanings and I don't feel like inventing new words when I can simply add adjectives as appropriate.
"Islamism" certainly does not mean "Islamic terrorism". It means a belief that Islam should be a force in the public sphere, not just a private religion.
Well, it's certainly a pity that Islamic terrorism is a big enough threat that they had to invent a Huffman-coded word for it. Though perhaps we can treat it as a relief that Christian terrorism is no longer so prevalent that we had to make a shorter term for it after we invented 'terrorist'.
Though I hear 'Crusader' is a close equivalent among many Muslims, for obvious reasons. I don't think Americans or most West. Europeans would understand that as a negative connotation though.
Christian on the other hand is already overloaded with many meanings and I don't feel like inventing new words when I can simply add adjectives as appropriate.
Pray tell, is my explanation sufficient?