Just give her ..her space for now ..hopefully she will come around
It could be the reason, there could be plenty of reasons. But like said above, give her some space. She will come around. Sometimes people need some time to process and then get excited. A friend of mine, her mom was not excited about the pregnancy but once baby came she is full on grandma mode, and super excited.
You didn't let her down at all. She's probably going through a lot emotionally and doesn't know how to process it. She will get there. Just don't push too much on her in a short amount of time.