Probably because you built your character to be better with 1 handed weapons than 2 by taking the dueling fighting style.
For comparison the average damage of longsword with dueling is 6.5+STR and with great weapon fighting is 6.3+STR. But greatsword with great weapon fighting is 8.3+STR (7+STR without).
Why is my Longsword 1d8+5 damage when used one-handed, but only 1d10+3 damage when two-handed?
One-handed is 6-13 damage, but two handed is 4-13 damage, thus making one-handed use better than two-.
I have STR 15.
Do you have the Duelling fighting style perhaps?
Probably because you built your character to be better with 1 handed weapons than 2 by taking the dueling fighting style.
For comparison the average damage of longsword with dueling is 6.5+STR and with great weapon fighting is 6.3+STR. But greatsword with great weapon fighting is 8.3+STR (7+STR without).
Thanks DxJxC! I'm new to 5e :)