He may not be the most popular character, some people don't like him. Maybe he can be viewed as that "Bruce Lee clone" later ended up more of a Chinese version of Street Fighter's Ryu. So what's with him? I felt like some of his moves having "dragon" with them, his Dragon fatality, his MK Arcade Drop fatality, his MK3 friendship and his current promotion as ruler of the Netherrealm made him an important mascot. I wonder was his plotline where he and Kitana got promoted to replace Shinnok in the Netherrealm meant to give him another mascot role? Granted, Mortal Kombat as of current is under Netherrealm Studios. Plus, after he was supposedly killed in Deadly Alliance he makes a major return in Mortal Kombat Deception, he wasn't missed out in MK vs. DC and he's back for MKX as Netherrealm's version of Evil Ryu. Too bad though, they didn't give him a few more moves to show his evil side.
If Liu Kang went from Bruce Lee to Ryu, I felt like Kung Lao is the Ken Masters of the series in some way. Okay he didn't appear in any MK Media outside canon except MK Legacy but I can't deny the guy seems necessary. He was missing in MK4 but was returned in MK Gold but I don't know why he didn't return in MK Unchained. Even if he's less popular but still, I felt like he's part of this list.