Why wasn't Cain mentioned in Adams generations?

Because Cain was and never will be part of Adam's line. He is the son of the serpent in the garden who seduced Eve. Google the serpent seed line to read more on this subject. That's why he could so easily kill Abel. This is a complex and deep subject that most preachers avoid it like the plague. So read up and learn.

because Adam had more than just 2 children. the Generation listed has to do with the line that lead up to the Messiah. since Cain was a murderer, he was not part of that Line. LUKE chap 3

The Generations of Genesis were written separately from the rest. They were meant to tie all the stories together. Look into the Documentary Hypothesis.

I hate using Wikipedia but this is a pretty good explanation

http://en.wikipedia.org/wiki/Documentary_hypothesi...

Luke 3; 38 [son] of E′nosh, [son] of Seth, [son] of Adam, [son] of God.

Seth was the third son of Adam. and from Seth came all of mankind. Cain, the first son, murdered Abel the second son. Because of this act. he was regarded by God as unworthy of any honour.

The Bible does not record the time of the death of Cain,

But it does record what became of him.

Genesis 4; 13 At this Cain said to Jehovah: “My punishment for error is too great to carry. 14 Here you are actually driving me this day from off the surface of the ground, and from your face I shall be concealed; and I must become a wanderer and fugitive on the earth, and it is certain that anyone finding me will kill me.” 15 At this Jehovah said to him: “For that reason anyone killing Cain must suffer vengeance seven times.”

And so Jehovah set up a sign for Cain in order that no one finding him should strike him. 16 With that Cain went away from the face of Jehovah and took up residence in the land of Fugitiveness to the east of E′den.

17 Afterward Cain had intercourse with his wife and she became pregnant and gave birth to E′noch. Then he engaged in building a city and called the city’s name by the name of his son E′noch. 18 Later there was born to E′noch I′rad. And I′rad became father to Me·hu′ja·el, and Me·hu′ja·el became father to Me·thu′sha·el, and Me·thu′sha·el became father to La′mech.

Because after he commited the first murder, he was considered the first family outcast. You know.. that guy you just don't mention at family get-togethers.

Because he was cast out after Abel's murder. The descendants traced through that genealogy are those of Seth.

Because :

a# he wasn't a good man.

b# he was the first murderer. he killed his own brother.

c# he didn't obey God's commad.

Because Neither of them exist.