"Can anyone let me know why?"
Because a literal "i" in the variable name is not automagically updated using the value of some variable that happens to be named "i". Imagine the chaos that would ensue if that were the case.
Forcing pseudo-indices into the variable names forces you into writing slow, complex, inefficient, obfsucated, buggy code when you try to access your data in a loop:
In contrast, when using actual indices your code will be simpler and much more efficient. You should use indexing.
You have not told us the most important information: how did you get all of those variables into the workspace? I doubt that you named them all by hand...
