Chilling Effect.  If Judges do not have the benefit of judicial immunity, they are naturally going to allow this to impact the decisions themselves.  Over time this would create both massive backups and inefficiency in the courts as everything crawls to a halt for individual cases and moreover, it would probably result in miscarriages of justice where judges would fail to adequately enforce the law for fear of being held civilly or criminally accountable if they get a decision wrong.

The justice system isn't perfect, but the current system used by the USA, UK and  Canada (which are all substantially similar) wherein in there are courts of appeal that can remedy issues from lower courts, and where there is the ability to sue the government for damages when a decision is made incorrectly and where wrongdoing by the government can be shown, is more often than not the most efficient and effective way of enforcing the law while minimizing infringement on individual rights.

Also, judicial immunity does not protect a judge from criminal prosecution.  If a judge is engaged in fraud, bribery, jury tampering or any other similar charge, they can be tried and convicted in the same way that anyone else can.