Merrick Garland, serving as the Attorney General of the United States, operates within the framework of the law and legal procedures. Charging an individual, especially someone as prominent as a former President like Donald Trump, involves a thorough investigation, gathering evidence, and ensuring that the case is legally sound.
Where he finds the lawyers to get away with what he does he does, knowing he's out of money and lawyers, is beyond me. That said, the timing of charging someone with alleged crimes can be influenced by various factors, including the complexity of the case, the amount of evidence available, legal precedents, political considerations, and ensuring that the case can withstand legal scrutiny.
Charging individuals with crimes are typically made based on legal merits [Remember: DJT is a master at plausible deniability and being schrodinger's douchebag.] and evidence rather than external pressures or political motivations. Prosecutors *must* adhere to *strict* legal standards and ensure they have a solid case before proceeding with charges.
To make another example, Jack Smith urged SCOTUS to make their decision expediently as this is a matter of national security and is a fundamentally constitutional crisis by what the defense claims.