Why did Derek Chauvin refuse to testify in court regarding George Floyd's murder?

Because there was no defense for what he did.

The prosecutions evidence is pretty irrefutable.  What could he say to defend kneeling on the man's neck

continuously for 9 minutes and 29 seconds until he stopped moving.  4 minutes of that was 

AFTER HE WAS DEAD.

How many times did they say "use of force" and 

"REASSESSING  the situation"?  Clearly there was no "reassessing" on his part if the guy was dead for 4 mins.

The dumbest thing anyone could ever do is testify at their own criminal trial, even if they are the most innocent man in human history. 

Probably he has a lot to hide.

As not to incriminate himself. 9 minutes & 29 seconds with someone putting a knee on your airway is hard to defend.

Strategically speaking, every witness called has to be weighed in terms of what positive evidence they bring for your side vs. what it allows the other side to bring in. If Chauvin testified that would give the prosecution the ability to bring up everything Chauvin has ever done. They would go through every mistake he ever made in his career, every parking ticket he's gotten and the grades he made in 3rd grade. I guarantee at least 1 of their 14 lawyers has spent the last 10 months preparing for the possibility of Chauvin testifying. He would probably be on the stand for at least a week. That's not how the defense would want to end the trial. 

So, I don't see that he brings much positive evidence to the case but it would bring a lot of negatives.  Maybe, if the defense was losing they might call him but the way this trial is going that would be dumb, 

So it was purely a strategic decision. 

This falls under:

Anything you say CAN and WILL be used against you. Pleading the 5th Ammendment ( choosing to remain silent)protects people from self incrimination.

Even criminals have rights.

It is every defendant's right to not be a witness against themselves by subjecting themselves to cross-examination.

If you have ever had the experience of being in court then you know that an experienced prosecutor can turn things around on you with each statement you make. Hence making you seem guilty when you’re not.

I believe the great President Trump claimed that if you ever have to plead the 5th that you are guilty. 

Here's one opinion.

The burden is on the state to prove he committed a crime. He is not obligated to prove he is innocent. That is a right provided in the Fifth Amendment.