And let's look at the logical thought process here. If (and it's a big "if") Trump obstructed justice, whose investigation would be obstructed? Mueller's.
Okay, so Mueller declined to prosecute the obstruction. Well, then whose responsibility is it to prosecute obstruction against Mueller's investigation? Now, I'm not a lawyer, but I think it's safe to say that if the ball is able to be picked up by somebody else, they'd have a heck of a time trying to convince a judge that there's a good reason to attempt to indict a sitting president (which, btw, can't be done anyway) on charges that the offended party declined to pursue when they had every opportunity to do so.
If your neighbor suspects that their uncle else broke into their house but didn't have enough evidence to prove it and thereby declines to press charges, you can't take over the "open investigation" and press charges yourself just because you really don't like your neighbor's uncle.