He's probably wary of adding perjury on top of the charge of lying to the FBI

Michael Sussman won’t take the stand

I think it’s highly improbable that a Washington, D..C. jury will convict him, especially when the deciders of his fate were drawn from a jury pool assembled in a city that went 95% for Biden, and there are several jurors who have admitted contributing to Biden’s election campaign. But if Sussman does take the stand and is somehow convicted, even this pro-Biden judge might punish him by enhancing his sentence for lying to the court: a judge did exactly that to Juicy Smollett, and though the man will never serve a day of that sentence, Sussman might be nervous about taking that risk.

And of course, he’d also have the daunting, near-impossible task of persuasively explaining away his damning behavior, and emails.

But juries expect defendants to testify, and notwithstanding all the instructions from the presiding judge that they should draw no inference of guilt from a defendant’s refusal to tell his story under oath, they do.