Character Analysis
The point of Lennie and Bailey's mother is that she's a mystery. They don't know anything about her beyond that she has the family "restless gene" and left them as babies so she could explore the world. Young Bailey and Lennie made up stories about her, and Paige was something that they shared.
When Lennie learns a few tiny facts about Paige (that she cooked pesto with walnuts and loved lilacs) the idea of her as an explorer was shattered because she becomes a real person. And knowing a real person abandoned you isn't very romantic.
The real purpose Paige plays, though, is to isolate Lennie when Bailey dies. Because her mom is already gone, when her sister disappears from her life, too, Lennie feels that much more lost. A big part of the work she has to do is open her eyes to seeing Gram as the mother figure she's always been to her—in other words, with Paige on the scene, we'd have ourselves a totally different book to read.