That's actually forming another good question. Since they (Eddie and Jake) did die at different times, on different levels, how did they become brothers in the new NY???

In reality, maybe King thought it would seem too "convenient" to just kill everyone off so Roland could reach his tower alone. This was one more way to remove a member of the ka-tet--and at the same time foster a reunion of sorts on the other side.

Your theory of her renouncing the tower would definitely be a deeper answer to the question "why?".