why dragonite DT-HB is better than DB-HB??

Dragon Tail has both higher DPS and EPS compared to Dragon Breath. If you're spamming attacks, then it's better in every way. The tier lists don't take into account dodging.

Field test it.

I don't think too many people would say DT/HB is better than DB/HB. DT/O is so strong because of Outrage, if we could get a DB/O then DT/O wouldn't be rated A imo.

DB is just better on attack.