They can't start up new Nobel prizes; the six they have at the moment are specified in Nobel's will. That's partly why they don't have one for astronomy and astrophysics even though it's a major research area these days.
The following (written in 2005) might help explain why the Nobel Foundation is awarding the prize for environmentalism and conservation these days - seems as though some of the original criteria are rather outdated:
"In his last will and testament,
Norwegian industrialist Alfred Nobel instructed his estate's executors to bestow a cash prize "to the person who shall have done the most or the best work for fraternity between the nations, for the abolition or reduction of standing armies and for the holding and promotion of peace congresses."
NPR : The Nobel Peace Prize and its Past Winners