Because when you're just detained, there's still hope you can get away without a search. "He's got no cause to search the car, if we just shut up and play it cool, we're good." Why on earth would you get violent then?
When the dog starts barking...that's it. The drugs now not just might be found, the drugs are detected, and the vehicle will be searched, and you are going to jail. Do you not think that could be the trigger that pushes someone over to violence?
Regardless, I don't think there was reasonable suspicion for a delay (for safety or drugs). "Reasonable suspicion" is a legal standard that allows for brief detainment and investigation. It requires articulatable facts about a specific person at a specific time that would arouse reasonable suspicion in a reasonable person. It's the same standard by which anybody gets pulled over. "I saw that car swerve, I have reasonable suspicion that guy is drunk." So you pull him over to see if he's drunk.
Now, a dog sniff does not (currently, I think this will be reviewed in coming years) count as a "search" because it (presumably) does not reveal non-criminal activity. So there's (currently) no 4th amendment problem running the dog. However, running the dog has nothing to do with the primary mission of traffic safety. The vehicle was pulled over for traffic violations, not upon suspicion of drug trafficking.
So while you can run the dog without RS of anything (the dog's nose essentially counts as "plain sight" for the officer's eyes), you can't delay "because drugs" without RS of drugs. And not even safety, because first there's no RS of danger, and even if there were, it could only apply to the situation for which RS exists (the traffic infraction).
So the only way the delay would be acceptable would be if the officer had RS of drugs, and I don't think that can be successfully argued. The only thing he had was a fishy story from the passenger about coming back from seeing a car to buy at midnight. Which is weird. But that does not generate RS of drugs. So the officer just had a hunch (which was totally correct! Dudes were running meth. Good instincts), but that's not the same as RS, and therefore not enough to delay.
And that's the question, that apparently has not been satisfactorily answered. Did the officer have reasonable suspicion of criminal activity that would justify the delay? And the 8th Circuit did not address that issue. The Supreme Court did not decide the case in favor of Rodriguez...they remanded it back to the 8th Circuit to address the question of RS. My guess is they'll say "no, RS didn't exist in this case," but we'll see.