Here's why:
* Lame (adjective): Meaning "disabled or injured, especially in a leg or foot." This is the meaning used when describing a walk or step that is impaired.
* Lame (adjective): Meaning "uninspired, weak, or unconvincing." This meaning is unrelated to walking.
While the word "lame" has multiple meanings, the context of "walk or step" clarifies that it refers to the physical disability.