Yoda & Obi-Wan tried when the Empire was just getting their legs under them. Obi-Wan did his part by taking out Vader, or so he thought
Yoda snuck into Sidious' chamber, who didn't even know he was still alive. After their fight the place got flooded by troopers. At that point they lost the only chance they had, the element of surprise.
Now the whole galaxy knew Yoda was alive and there would be an army looking for him wherever he stepped foot, there wasn't going to be an unobstructed path to Sidious again. Let's not forget Yoda was
, so who knows how much that fight took out of him
They didn't know if there were even any other jedi left or where they would be, no army to go against the troopers...So they had no way to fight the war until the element of surprise was available again
No excuse for Luke however. None.