I think any drama/movie/tv show becomes popular when it has a good plot and actors. Same goes for the untamed.

PLOT is such that it makes you feel alot of emotions at the same time. Its very intelligently written.

ACTORS are just mind blowing. It looks like every actor was born to play the character they played.

MUSIC is soulful especially the main theme song, character songs of the two main characters and the character song of jiang yanli (I am assuming you know the characters)

OUTFITS are just so beautiful. The untamed is based in ancient China and the characters wear traditional dresses which adds to the popularity as well.