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


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.)
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.)