Why do parolees have even more diminished right against searches and seizures than probationers?
The reason for this is that parolees have, by definition, committed more serious offenses (or just more offenses) and are therefore more of a risk to society. In addition, they are still under a prison sentence and therefore have a reduced expectation of privacy.
Parole is granted at the end of a prison term. A parolee is let out early because of factors like good behavior. But the fact remains that the parolee has been in prison because they have committed serious and/or numerous offenses. This is in contrast to probation, which is granted instead of prison to someone who has committed a non-violent offense and/or has no prior criminal record. Therefore, a parolee is deemed to be a greater risk and more in need of supervision.
In addition, the parolee is still under a prison sentence. The Supreme Court has ruled that parolees are still under the legal custody of the prison system even if they are out of direct supervision. This means that they have much less of an expectation of privacy. Their expectation of privacy is more like that of a person who is actually incarcerated. For these reasons, parolees have fewer rights against search and seizure than probationers do.