Avot Derabbi Natan 9:2, commenting on Bamidbar 14:22, enumerates the 10 times that B'nai Israel tested G-d in the desert. The Mishnah states that G-d did not punish them until they spoke lashon hara about the land of Israel.
This implies that speaking lashon hara was even greater than the sin of the Golden Calf which was idol worship.
I mention that sin specifically because we see similar phrasing in Moses plea to G-d for forgiveness, such as mentioning that Egypt will think that G-d pre-planned to destroy his people in the desert. We also see a partial repetition of the 13 midot.
So G-d forgives the sin of the Golden Calf but not the sin of Lashon hara. Why was this sin unforgiveable?