Other countries get rid of dictators, why can't we?

[Ocelot II]

3. I think it did have to do with trusting good intentions.

The whole system was based on the assumption that there would be enough good-faith actors in all branches to check anyone who stepped out of line. Most of the time that arrangement has worked pretty well. What they apparently didn't anticipate was the possibility that all three branches could be infiltrated or coopted to the point where a bad actor in the executive branch could take control.