1. Guessable, but not knowable
I think, the truth is that no one knows for sure, as many documents about the subject remain classified. Even if some are unclassified, we don't have any guarantees that the the unclassified documents are exclusive.
The only tool we have to analyse this situation, is to logically analyse the history of the involved parties, their current behaviour. Then use such analysis to generate educated guesses about what might be going on.
2. Others' guesses so far
I've read the other answers, and the comments underneath them. They're all contradictory. Here is a summary:
- Because OBL is dead — This is contradictory, as OBL was alive until 2011, which is a decade since 9/11. Is a decade not enough time to charge OBL for his crime? OBL was publicly accused for 9/11, and the U.S. Navy SEALS was assigned the duty to assassinate OBL for 9/11, but somehow not accused legally.
- Because the evidence was too weak for a trial — If the evidence is too weak for a trial, then why use it to not only assassinate him, but even wage a war against an entire country (Afghanistan)?
- Because the 9/11 attack was big — So? Then perhaps charge him legally with a big penalty for his big crime?
- Because OBL was not in USA, otherwise he could've been convicted like Khalid Sheikh Mohammed —
- Irrelevant, as criminals get charged for their crime in a nation, irrespective of their physical location. Sometimes INTERPOL is used to bring them to law.
- The case of Khalid Sheikh Mohammed is not a good case of legal trials, as the suspect admitted under torture. U.S. lawyers defending other suspects have mocked some of the trials, concerning other suspects that were detained in the same facility, as "hearsay evidence".
- Might not be that important in this context, but Khalid Sheikh Mohammed is still not convicted (detained for 18 years).
3. My best guess so far
Based on my analyses, my best guesses (which is all we can do at the moment) is that USA intentionally desires to keep the 9/11 vague in order to continue pressuring other governments, for strategic gains, beyond the lifespan of individuals.
E.g. if USA announces the full list of the suspects, convicts them, then that's the end of it. But if the case is not resolved, then USA can, say, use the case to continuously pressure Saudi (which USA keeps doing until today) irrespective of who is the king in charge.
What might support this conclusion is that, nowadays, USA pressures Saudi more than it pressures Taliban about 9/11, despite the fact that Saudi had considered OBL an enemy of Saudi and revoked his citizenship since 1994 (7 years before 9/11). Logically, I cannot find any better guess than: pressuring Saudi is more profitable than pressuring Taliban. If Taliban's Afghanistan becomes richer, then they may get the pressure.
Appendix: The bigger picture
USA must not be hated for this. We must not forget the bigger picture, that every entity commits mistakes, and USA's share of mistakes seem to be relatively small specially when considering that it is the strongest world power history has seen so far. Entities that use such events to hate USA are often puzzled with worse mistakes (specially when you consider that their powers are far less).
The whole sequence of incidents, starting from 9/11 and going through years of wars in Afghanistan and Iraq, was to the best of my observation a mistake that only benefited China/Russia/Iran in the short run (but will harm them in the long run), and harmed everyone else.
The fundamental idea of invading Afghanistan and Iraq may not have been necessarily a mistake, but the mistake was in the details: handing those regions to Iran's allies, which simply empowered Russia and China against US and NATO interests, which their effects echoed not only in Middle East, but also in Europe (Crimea) and Asia (Hong Kong, etc).
This is unfortunate, as Taliban used to be a force against Russia (pro Chechnya independence) and could've made a strategic US ally. This is also a loss for Afghanistan as China continues to enslave the Uyghurs.
Maybe if this sequence of incidents wasn't done 20 years ago, today Australia might not be in a position to need nuclear submarines to guard itself against China's threats.
To the best of my observation, what happened over the past 20 years was a demonstration of the death spiral bug in the game theory principle tit for tat. Perhaps this is a remainder to use the solution: tit for tat with forgiveness to advance our civilisation for the greater good of life forms. Hating USA will simply continue the death spiral issue, and will lead to everyone's loss. We need to be realistic, and exercise forgiveness, specially that the intentions of USA/NATO are towards doing more good.