There are two other reasons (cited in the Poskim) on why a person who commits suicide than a murderer.
One reason is that death is the ultimate atonement for one’s sins. If it is through that method however, that he sins, it cannot serve as a Kaparah.
The second reason is quite an interesting one. The Shiva etc. is only so that the family should be bereaved. A family that a loved one committed suicide cannot be deprived from their pain therefore the customs do not apply.