Federal grand jury testimony is kept secret due to grand juries operating under far less strict legal standards and proceedings than a typical jury trial, as their primary duty is not to convict an individual but to grant authority to the US Attorney General to pursue charges against an individual, as per the Fifth Amendment:
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury…
The grand jury panel is led by the prosecution, not an impartial judge. The defendant is forbidden from presenting their case against the prosecution; in fact, the defendant will frequently not even be notified a grand jury has been convened against them. The grand jury is instructed to return an indictment if 12 of the 16-23 members find "probable cause" of a crime, not the "beyond reasonable doubt" standard of a typical trial where 12 jurors must unanimously agree.
This means, in their duty to grant authority to federal government to pursue charges, the grand jury is inherently one-sided. The defendant does not have the opportunity to defend themselves until the regular trial, at which point any relevant information used in the grand jury panel will have to be re-introduced to the case as part of normal legal proceedings.
The preliminary and non-enforceable nature of grand jury panels was part of the rational for the Supreme Court upholding the secrecy of grand juries in Douglas Oil Co. of Cal. v. Petrol Stops Northwest in 1979. Justice Powell writing for the majority stated:
First, if preindictment proceedings were made public, many prospective witnesses would be hesitant to come forward voluntarily, knowing that those against whom they testify would be aware of that testimony. Moreover, witnesses who appeared before the grand jury would be less likely to testify fully and frankly, as they would be open to retribution as well as to inducements. There also would be the risk that those about to be indicted would flee, or would try to influence individual grand jurors to vote against indictment. Finally, by preserving the secrecy of the proceedings, we assure that persons who are accused but exonerated by the grand jury will not be held up to public ridicule.
There are court cases prior to that ruling that do allow for the disclosure of grand jury transcripts with certain restrictions, such as the 1958 case United States v. Procter & Gamble Co. which allows a private party to obtain transcripts if their defense would be greatly prejudiced without access to those transcripts, the 1966 case Dennis v. United States which found the First Amendment protections that enables grand jury panel witnesses to publicly discuss their testimony allows for public dissemination of their testimony, or the Jencks Act which releases grand jury testimony of a witness to the defense after said witness testifies in open trial, however these disclosures primarily deal with the use of grand jury testimony in further legal proceedings, and do not yet include any release of on the basis of public interest alone.