Why hasn't a tribunal or case been opened to investigate this
How do you know that this isn't being investigated?
Investigations often aren't made public until they are concluded and charges are brought.
Who decides whether or not this amounts to bribery or has colored his
decisions? Certainly, if so, this would be illegal, right?
Ultimately, a jury in a criminal case (together with appellate court judges reviewing the decision), would decide if this amounted to criminal bribery. In an impeachment case, it would be decided by members of Congress.
While actual bribery would be a crime, conduct that is merely unethical for judge that could have colored his decisions or created an appearance of impropriety without rising to the level of bribery is not a crime.
And, a U.S. Supreme Court justice is basically immune to any penalty or punishment (other than impeachment), based upon allegations of conduct by a justice that violates canons of judicial ethics but does not constitute a crime.
he is subject to the laws of any other appointed bureaucrats, right?
He is not.
The mandatory financial disclosures in the U.S. federal judicial branch for judges have direct teeth primarily by virtue of these violations constituting violations of judicial ethics rules for judges. It is not a crime for a judge not to make a mandatory financial disclosure.
But, U.S. Supreme Court judges are not, in general, sanctionable for violation of judicial ethics rules by any body with an ability to enforce its directions against them.
The Attorney General might be able to bring a criminal charges under a general federal bribery statute, but only if the elements of those crimes are met, and only after an investigation of the facts which in this case was only recently known due to investigatory journalism.
The primary federal bribery statute is 18 U.S.C. § 201. Section 201(b)(2) and (c)(1)(B) which applies to public officials who receive bribes has the following operative language:
Whoever - . . .
(2)being a public official or person selected to be a public official,
directly or indirectly, corruptly demands, seeks, receives, accepts,
or agrees to receive or accept anything of value personally or for any
other person or entity, in return for:
(A)being influenced in the performance of any official act;
(B)being influenced to commit or aid in committing, or to collude in,
or allow, any fraud, or make opportunity for the commission of any
fraud, on the United States; or
(C)being induced to do or omit to do any act in violation of the
official duty of such official or person; . . .
shall be fined under this title or not more than three times the
monetary equivalent of the thing of value, whichever is greater, or
imprisoned for not more than fifteen years, or both, and may be
disqualified from holding any office of honor, trust, or profit under
the United States. . . .
(1)otherwise than as provided by law for the proper discharge of
(B)being a public official, former public official, or person selected
to be a public official, otherwise than as provided by law for the
proper discharge of official duty, directly or indirectly demands,
seeks, receives, accepts, or agrees to receive or accept anything of
value personally for or because of any official act performed or to be
performed by such official or person; . . .
shall be fined under this title or imprisoned for not more than two
years, or both.
Basically, a criminal prosecution for bribery requires proof of a quid pro quo.
The U.S. Supreme Court significantly narrowed the scope of federal criminal statutes for prosecuting official corruption short of actual quid pro quo bribery on the grounds that the public is denied the honest services of a public official in the case of McDonnell v. United States, No. 15-474, 379 U.S. ___ (June 27, 2016) (although the federalism oriented basis for the decision is arguably inapplicable in a case against a federal judge).
Also, if Justice Thomas had the support of a majority of the non-conflicted judges on the U.S. Supreme Court (which is plausible at present which Justice Thomas is part of a six judge conservative majority out of nine judges), the U.S. Supreme Court could find a reason to declare Justice Thomas immune from the charges or otherwise dismiss them.
Congress could, of course, impeach Justice Thomas for this conduct (impeachment grounds are nonjusticiable, i.e. not subject to court review), and has impeached lower court federal judges for corruption related charges before. But that too takes time and the recent accusations against Justice Thomas don't have sufficient bipartisan support to clear a Republican controlled U.S. House of Representatives and a razor thin majority of Democrats and Senators who caucus with them in the U.S. Senate.
The lack of bipartisan support is in part, as noted by JoeW in the comments, because if Justice Thomas were removed from office, the vacancy created would be filled by President Biden with a far less conservative justice than Justice Thomas.