Why are some people taciturn and don't say much? I often have more to say about certain topics than others. Why do some people find it difficult to talk?

For example, if someone asked me how was your vacation in…, I'd have a ton of ideas. I'd talk about it in detail. Some people just have standard answers like, "Yeah, he was good!" or something similar. Why on earth do some people say so little? Why is that?