There's several reasons why it's taking a while, although, compared to Watergate, this has been like a flash:
1. You can't just up and use foreign intelligence in the prosecution of US persons. This presents Mueller with a significant bureaucratic problem.
2. The scope of the inquiry is so large, the individuals involved so many, that this is going to take time. As I've said, years from now, people will still be getting arrested and tried for their involvement in 2016. Literally dozens, if not hundreds, of people involved. And that may just be the Americans.
3. Mueller only gets one shot at this, and he can't screw up. If he fails here, we risk losing this nation and everything we hold dear. If any of these individuals walk, we may eventually see their crimes become the "norm."
4. Since 20-25 percent (the hardline Donny base) truly are deplorables, smart strategy must come into play in terms of how this is publicly revealed. Given Team Treason time to publicly hang themselves, while not a foolproof method, does at least help mitigate the chances of deplorables doing incredibly stupid things once the truth comes out.
5. We're in uncharted political and legal waters. Honestly, perhaps the biggest holdup is simply Mueller's team having to figure out how they can indict and charge a sitting president with a congress that is complicit.
That's just a few reasons why. I'm sure there are likely others.