Because the cases need to make their way in the courts. Once pardoned, all cases are dropped.

But that is not the only reason, however. Optics are important as well. As long as the Capitol Riots are presented as Right Wing insurrection by the mockingbird, he cannot endorse it. If he simply pardons them, the narrative will be solidified and Trump will be smeared as supporting extremism, even by his own supporters.

(You will be surprised at the present how many Trump supporters indeed think Qanon extremists caused the violence on Jan 6th).