People have different interests, different personalitites and as a result, classes are formed of students with different abilities. All of the factors mentioned are causing mixed ability. If I had to put them in order, it would be like this:
1. previous experience
2. motivation
3. attitude
4. aptitude
5. personality
6. learning style
As it can be seen, I think previous experience and motivation are the most important factors in causing mixed ability. The progress a student makes depends on what he learned and the manner in which he learns depends on how he is used to learn.
In what concerns the fact that mixed ability is considered a problem, I strongly disagree. There are obvious advantages in having students of different abilities in the same class. First of all, this is a challenge for the teacher, who has to adapt both the methods and the materials to the students' needs. But, more importantly, in a mixed ability class students can learn from one another, they can explain to one another what they do not know, and I think the impact is bigger.