The Mueller investigation hasn't been running very long at all. And has plenty of results for its relatively short duration to date. These kinds of things typically take years.
This is just completely contrary to basic criminal investigation. You don't lay charges until you have either uncovered all the evidence you can on the target, or you have enough evidence to apply pressure and hope they'll offer you some new evidence in exchange for a plea deal. Organized crime investigations don't start by charged the boss, they start by nabbing underlings and mooks, building up to lieutenants, and eventually building enough evidence to charge the boss for everything he's responsible for, or at least enough to ensure conviction on something (like Capone and tax evasion).
If Mueller thinks there's any possibility they'll find more evidence against Trump, he's going to hold off on filing charges until he gets it.