The drama averaged 4.3% (AGB Nielsen) nationwide, which isn't great. Many people have blamed Jang Geun Suk for the low ratings, saying that he needs to tackle different types of roles. Sure, he has played the emotionally unavailable jerk who eventually charms us with his cat smiles, and ends up intensely invested in love, many times. He's good at it, and we knew that going into Pretty Man.
The problem was that the writing completely failed to deliver a plot that had enormous potential. If the script had allowed for more than minimal character development, had dug deeper into the nature of relationships, and had made connections within the story instead of abbreviating everything and then glossing it all over, viewership would have been higher, and everyone would be saying that Sukkie was awesome.