Why won't she talk about it anymore?

My mom said that she'll get me an iPhone on Saturday. But when I start to talk about it because I'm excited, she shuts me off and doesn't make me talk about it anymore. So i don't know why she won't talk about it anymore.

She was probably joking about it

She could have had a sudden financial problem that makes it hard for her to get it for you

If she said she would get it for you, she meant it. But obviously she has realized that she can't afford it right now. Just be patient because she will eventually get you one.