The first such reason (and, as it turns out, the right one) is that said grandchild might just be a boy. As all the strips that have Mike yearn to be covered in filth, think of housework as being beneath him and play with scary guns because that's just what boys are into indicate, Lynn doesn't seem to be able to relate very well to anything male. As the strips that have Elly try to dress Lizzie up like a little doll-girl and those that have her smolder in rage because April resists said tendency indicate, what Lynn seems to want is a little girl who'll sit quietly as she expresses herself creatively owing to a perceived lack of a gal pal to do girl stuff with growing up. Boys don't afford her that opportunity so are stinky.
The second idiotic reason would have been why April is gross-buckets, why Lawrence is all wrong and why the Nichols family was foredoomed to fail: Katie and Lane decided to present her poor mother with another girl despite the fact that proper families have to have two children, one male and the other female and both at each others throats fighting for their parents' love like God intended. Any indication that Lynn's insistence that any sort of deviation from her past is not the worst thing ever is not on because we're dealing with a rigid thinker.
The third reason that comes to mind is that we're also dealing with someone who gets bored easily. If this is the case, we're dealing with her not saying a heck of a lot because the novelty has worn off and being a grandmother is just another disappointing chore that takes too long and is too much like work.