Historically to prevent pre-contamination and erroneous biosignature readings. This may ease once we have some solid pointers or confidence there may be none (caution is likely warranted due to the gravity of such a discovery).
Consider Mars. Endless probes for 50 years going to Mars looking for life. No clue of life has ever been found. At what point do we face the fact that Mars is a dead rock?
What we should be doing is collecting samples of extremophiles from the Earth, and attaching a few packets of them to every probe going to Mars, and see what happens. Probably nothing will happen, but it's worth a try.
