Merrick Garland became AG on March 12, IIRC. The DoJ has a huge backlog of work, not least the Jan. 6 insurrection which has already resulted in charges against more than 100 people with more being added regularly. This is one of the most complicated investigations and prosecutions ever. While trying to clear the cases of individuals who got caught up in the riot, investigators are untangling the leadership, relationships and actions of the organized groups who came to the Capitol armed and with a specific plan to stop certification of the election and harm or murder members of congress.
Meanwhile, the SDNY is investigating the Trump organization's finances. They just got access to the financial records and tax returns about a month ago, after DoJ obstruction ended. There are reported to be literally millions of pages of documents. They are also investigating Rudy Giuliani's actions in Ukraine, which are at least tangentially linked to the 2016 election -- some of the same players are involved -- and directly linked to what may be corrupt and illegal actions by members of the Trump administration.
There is a good deal more corruption that occurred in the previous administration, some of which has been publicly reported and even led to resignations, which has not resulted in prosecution. Then there is the case of Matt Gaetz, which seems fairly simple on its surface but there are rumors that it is more complicated. In any case they will have to sew it up very tight in order to proceed. All of this will take a while, and they will have to decide whether and when to follow up on Robert Mueller's referrals for possible prosecution. As a legal and practical matter, these will not be easy decisions and events will certainly not move quickly. Patience, grasshopper.