Well the legal system doesn't exist on paper. Laws are not what the legal system operates on. Allegedly, breaking a law is what allows, but doesn't require, the legal system to be involved at all.
So to answer your question, Sam Bankman is a nobody, that no one likes, and caused a very public number of people to lose a lot of money. His case is a slam-dunk, and has no further implications. Trump is a former president who did exactly the same shit that every other former president has done since Washington. So prosecuting him for his crimes now means that the power brokers of the US empire are now potentially open to prosecution. So of course the two aren't going to be comparable.
So there is no "official reason" because officially the judicial system is based on individual discretion.