Why did the Constitution specify that judges should serve indefinite terms during good behavior and that their salaries should not be reduced?
The Constitution specified this so that the judicial branch could be independent and could decide its cases based on the law rather than on political concerns.
If judges had shorter terms (and could be reelected or renominated) they would have that in mind as they decided their cases. They would make decisions while thinking about what decisions were most likely to make them popular. If judges had to worry about Congress reducing their pay, they would do the same. They would not be likely to make any decisions that would make Congress angry for fear of having their pay reduced.
In our system, the courts are supposed to protect our rights even when the majority wants to violate those rights. In order to do this, they have to be free from political concerns. That is why the Constitution protects them in the ways you mention.