Why do we have so much neurological research on and treatment designed for dyslexia, yet less so for dyscalculia?
This website http://www.aboutdyscalculia.org/causes.html provides answers to some of the questions you posed, although I am not sure it is the most up to date information. I would always advise one to do research on more scientific grounds.
From my perspective, dyslexia has also been more studied than dyscalculia as its impact in academics seem more easily perceived thus easier to diagnose. Furthermore, I think that there is a social component to the issue as people often accept with ease someone struggling with mathematics but respond with more preoccupation towards difficulties with reading and spelling.
I leave you one article that may be helpful to you. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3514770/