Why is the comparative structure of "not as...as" so much harder for ESL learners than the "-er/more/less than"?

edit

Hi there,

From your question, I can only suggest one logical solution: positive (affirmative) and negative sentences. Probably your students are not used to negative structures, thus they automatically avoid creating such sentences in their minds. I cannot see what your students' native language(s) is/are, but what I can tell from my personal experience is that some people tend to use affirmative and others negative sentences. This might also be a cultural feature, which could then explain their choice of an affirmative structure.

Hope this helps!

Regards, Filip