The best answer here is that it's because Game Freak wanted it that way. But, there are two other reasons I can think of that might explain Muk not having Poison Point:
1 — Muk can only have three Abilities. In Gen III it was given Stench as its signature ability, as well as Sticky Hold. Sticky Hold was also given to Gulpin/Swalot, two Pokémon similar in build to Grimer/Muk. When Gen V rolled around Muk acquired Poison Touch. There are many Abilities that fit with Muk's theme, but it can only have three of them. There's not enough room to give Muk both Poison Point and Poison Touch.
2 — So why does Muk have Poison Touch instead of Poison Point? The most logical answer to this question, methinks, is that Muk is not pointy. If you look at the list of Pokémon with Poison Point, you will notice that all of them have a spike or horn somewhere on their bodies (the exception being Skrelp). Poison Point works when the Pokémon pokes another Pokémon with its pointy bits. If you look at Muk, however, you will see it has no pointy bits. Muk is smooth and fluid, like a melted slushie. It doesn't really make sense for Muk to have the Ability Poison Point with that physique.
