Dorothy Bishop (of BishopBlog) has a nice PLoS paper looking at: Which neurodevelopmental disorders get researched and why?.
She took 35 "neurodevelopmental" disorders, ranging from rare genetic syndromes like Rett's, up to autism, ADHD and specific language impairment (SLI), and compared their prevalence stated in a textbook, to the number of scientific papers published about them over the past 15 years.
Note that with something like Rett's, there's no question that they're problems with brain development. With autism, some people would contest that but not many nowadays. With ADHD and some others, however, it's pretty controversial. Bishop includes them on the grounds that they're generally treated as neurodevelopmental in the scientific literature.
The graph above - which I should stress is mine; Bishop's are much less messy - shows the basic results.
First up, there's a correlation between prevalence and the number of research publications, but as you can see, it's pretty weak. Within the rare genetic disorders (pretty much everything below 0.1% prevalence) there does seem to be a relationship. When you get to the more common disorders, which are also the ones which are more controversial, there's no correlation at all.
Some points stand out:
Autism is very popular; it gets the same amount of research as intellectual disability aka mental retardation (ID/MR), even though ID/MR is 9 times more common (0.65% vs. 5.5%)
Down's Syndrome gets a huge amount of research despite being rare. It gets much more than Cerebral Palsy and Fragile X despite them all being severe and roughly as common.
Tourette's is much less studied than any other disorder with a similar prevalence.
In the bottom left you'll see a bunch of apparantly very common disorders like dyslexia, dyscalculia, and specific language impairment, which are extremely under-studied... if you accept those prevalence figures.
Bishop also notes that both the number of publications, and the amount of US government funding, for ADHD and autism research have skyrocketed in recent years. Especially ADHD which had just 356 publications in 1985–1989 but this increased nearly twenty-fold to 6158 in 2005–2009!
As for why all these figures are they way they are, it's less clear. Bishop discusses various factors like severity and the availability of funding in the paper, but this can't explain everything. It seems likely that some things are just more scientifically fashionable than others, for whatever reason...
Link: See also Bishop's Guardian piece about the paper.
Bishop DV (2010). Which neurodevelopmental disorders get researched and why? PloS one, 5 (11) PMID: 21152085