Here's why: There's no one in the country, other than Mueller (and maybe former White House counsel Don McGahn), who can shine a light on some of the trickiest questions involving Donald Trump raised by the probe -- most notably whether or not Trump committed acts of obstruction of justice.
And why is Mueller -- or at least his team -- worried about his potential testimony in front of a House committee on Capitol Hill? Because, politics. Writes CNN's team:
"The special counsel's team has expressed the notion that Mueller does not want to appear political after staying behind the scenes for two years and not speaking as he conducted his investigation into President Donald Trump."
HA HA HA. Dude, that whole "apolitical" ship sailed the second Mueller was named as special counsel by then-Deputy Attorney General Rod Rosenstein.
Or the moment that Trump abandoned the advice of his first set of lawyers and began savaging Mueller and the members of the special counsel's office on an almost-daily basis on Twitter.
Or the moment Mueller wrote a letter to Attorney General William Barr saying that Barr's topline memo of the broader report didn't fully cover the extent of the possible obstruction by Trump.
Mueller, and those advising him, are fooling themselves if they think that if he refuses to testify on Capitol Hill it will somehow preserve a patina of impartiality around him.
Now, it's also possible that the whole he-doesn't-want-to-be-political thing is simply cover for Mueller's discomfort with answering some questions that he knows would be coming his way from lawmakers. Questions like: Why didn't you offer a recommendation on obstruction of justice? If there had been no precedent that a sitting president can't be indicted, would you have charged Trump? Did Barr mislead the public about your conclusions in his initial four-page topline letter to Congress? And so on and so forth.
The Point: I still think Mueller testifies because he's too central to, well, everything not to.