The low fireball is what kills Reptile's forceballs. At a certain range, Reptile can't really put them on the screen before he's hit, and Kang recovers really fast. Spit's hitbox is really high up as well, so Kang can fireball underneath them pretty reliably, which sucks. This all opens him up to air fireballs that wouldn't be as big of a deal. Reptile's pretty much forced to play the matchup with the forceball aspect of it all basically gone. In a number of matchups, this isn't too big of a deal, because he can play around this, usually with his normals game. Unfortunately, Kang is stronger here as well. b3 beats out d4 well, and Reptile's other normals simply can't compare to Kang's at all. Also, b312 and f12 advance really fast, so Reptile's walkspeed is somewhat hard to abuse here. Also, because Liu Kang's d3 is 7f, and b312 and f12/f121 are all 0 on block, Reptile's forced into dashing to check the pressure, which is bad (The reason why this is bad is because punishing dash is a lot easier than punishing a d1 on block, so against someone with decent reactions, you're getting hit everytime your dash is blocked).
What helps Reptile out here is that Kang has awful wakeup. This means that Reptile getting a knockdown is huge, because this gives Reptile the opening he needs to get his forceball game into motion. And when Reptile's walkspeed works, it gives him this opening he needs. It's 4-6, but can feel like 3-7 at times if the Kang player is on point with his footsies.
