Because even as cain is alive and should have taken the position of the next oldest son, seth is considered one in the likeness of his father and takes the position of a son who brings forth the line. Even Eve notes this when she says, GOD has given me a son, because cain killed Abel. She acknowledges that cain is alive, but she does not acknowledge him as belonging any longer to the family line...
If you look at the genealogy in Genesis, each son mentioned is after his father's likeness...and Adam begat seth, in his likeness, and seth begat ________in his likeness. And so forth and so on.
Until we get to the table of nations, it is further stressed that physical order and anything according to physical flesh does NOT take any precedence over GOD'S call...
The sons of Noah mentioned as the one's who carried forth the genealogy, are noted not even based on their birth order, but because they are likened to their father as a son in the likeness of their father...
So, for instance, even as the sons mentioned are physically in order, the son to carry forward the genealogical line was not necessarily the first son...