In mental illness why they are rational with money not other things?

They're not. People with various forms of mental illness can be rational or irrational with any aspect of life. For example, a normally rational, responsible person with bipolar disorder can suddenly flip into a manic episode and rush off to vegas, gamble away their entire estate and get married all in one night. There's nothing special about money with mental illness. Careful treatment is the key.
Brain parts. Different parts of the brain do different things. Different brain chemical messengers or neurotransmitters do different things as well. The human brain is the most complicated structure in the universe.