Here's why:
* Success is a concrete noun, referring to the state of achieving a desired outcome.
* Achievement is an abstract noun, referring to the act of accomplishing something or the accomplishment itself.
You can use "achievement" to describe the process of becoming successful, or the result of achieving something:
* "Her achievement in graduating top of her class was a testament to her hard work."
* "They celebrated their achievement of reaching the summit of Mount Everest."