In general it's hard for the prosecution to bring up evidence of prior, unrelated convictions or wrongdoing. If you take the stand they are allowed to bring it

In general it's hard for the prosecution to bring up evidence of prior, unrelated convictions or wrongdoing. If you take the stand they are allowed to bring it up to impune your credibility as a witness though. Doesn't really apply to SBF, but it's an additional reason to those others have stated for anybody with a criminal past.