At Shavuos we were forced to except the Torah, accompanied by Thunder and miracles. These didn’t have a lasting effect (i.e we sinned at The Golden Calf), rather they wore-off.
This may be because when we go back to our normal “everyday” lives following a miraculous phenomenon there’s no thunder/lightening & we find it hard to see G-d amongst the mudane.
But at Purim we saw G-d’s Hand in the “everyday” events, and this had a much longer-lasting effect. Hence Purim is a greater day than Yom Kippur. Perhaps this is because we were now equipped with how to see G-d in the everyday “routine”.