Well, she may feel this situation was just dumped on her without her input. And, be honest: when your grandchild is over, how much of the scut work do you do? I don't mean taking her to the park or reading to her and other pleasant activities - I mean changing a diaper, dealing with her if she won't eat, etc. You know, the chores, which can often be messy and/or loud. If those are all or mostly falling to your wife, then she's probably none too thrilled to be going through an instant replay of new motherhood, 20-30 years later, when her back and knees hurt more, and she's long ago come to the conclusion that banging on pots isn't amusing.
That's not a lack of interest or affection - it's a person who wants their life back.
If you are doing the majority of the heavy lifting (perhaps even literally), then your wife has less of an argument, although she's still going to be woken up if the baby cries in the middle of the night, etc. But if you aren't doing the lion's share of this work, then it's about time you started.
If you want it to be easier for your granddaughter to be around, you're going to have to step up.
Oh, and PS, having other childcare arrangements (assuming they are affordable) is a good idea anyway. What if either of you get sick or injured? Or you want to go on an actual vacation? Your granddaughter would benefit from more interactions with other people - and it would be best if she met an alternate sitter long before you need the alternate sitter to step in.