Reasons for not releasing mark schemes for past exams?

On my course my university provides past exam papers but does not release mark schemes and I know this is the case for other universities. I am dyslexic which makes reading hard, and for my high-school exams I used exam papers with the mark scheme as a self-study method, (meaning I attempted questions and used the answers as a source of information on that topic). Which leads to what I see as one of the main arguments against not providing mark schemes; by providing exam papers you give students an idea of what they need to study but not the information they need to know (i.e. the answers) meaning they will spend time (which could be spent on learning other things) looking in text books for answers.

So what are the reasons for not providing mark schemes to past exam papers?

Edit

Just to clarify some points:

Firstly I am from the UK hence the phrase 'mark scheme'. When a wrote the question (which was a while back know) I guess I was more tending towards the idea of a mark scheme as example solutions rather then a 'grading rubric'. I must admit 'mark scheme' was probably the wrong phrase to use in the question. Just to keep the question consistent with the answers, the interpretation of a 'mark scheme' as "any method that allows the student to work out what they are expected to write in the exam" works best.

Also a while back I did email the relevant people at my university about the situation concerning 'mark schemes', also explaining about my dyslexia and how I learn best. They replied by simply saying that the review the situation every year, but gave me no further information for the reasons behind it.

Answer

I'm not aware of any particularly compelling reason not to provide solutions for past exams, but here are two possible reasons:

1. 
   

   Providing solutions encourages students to study by learning isolated facts. If they have to look things up, then they are more likely to absorb some of the context at the same time. (Looking things up takes more time, but you typically learn more than just the one thing you were looking for.)

   
   
   


2. 
   

   Some students find it very difficult to resist looking at solutions when they are provided. They will think for a minute or two about a problem and then check the solutions and convince themselves that this is exactly what they would have done if they had spent more time. In some fields (such as mathematics) working out a solution oneself is usually much more educational than reading someone else's solution. Withholding solutions is inconvenient for students, but the inconvenience may push them towards better study habits.

   
   

Only your professors can tell you why they do not provide solutions; without knowing the specific situation, all we can do is guess. It can't hurt to ask them and to explain why having solutions would be valuable for you. They may not have strong reasons and might decide to start providing solutions. (And, as Dave Rose points out in his answer, you should also get any appropriate accommodations for dyslexia.)