I think that's a perfectly reasonable question. The basic answer I'd give is that the bigger/harsher a crime is the longer it takes to build as a case.

The bigger or harsher a crime is, it usually has more protections and requirements in order to prove. It takes longer to gather the relevant evidence, longer to put together a theory of the case, and longer to button everything down and present to a grand jury.

It's kind of like asking why it takes developers longer to build the biggest/coolest features in a video games. It's a lot more work!

This was also a question that came up quite frequently when the first Jan 6th defendants were charged. The first wave of defendants were charged with things like "trespassing" and other fairly mild charges. A lot of people were upset about that. But they just came first because they were much easier cases to proves and make. Eventually more serious charges such as "assault with a deadly weapon", "obstructing congress" and "seditious conspiracy" were later charged and convicted in front of a jury.

So, I generally wouldn't be surprised to see the easier/simpler cases come before more complicated cases. That's not to say that he will be charged with bigger/harsher crimes. Maybe the facts won't bear those cases out, and they won't be charged. But the ordering doesn't seem like it should be particularly surprising.