Usually, the phrase is "not my cup of tea." It is not as common to hear things like, "Dancing is my cup of tea." It's more common to hear "Dancing is not my cup of tea," meaning the speaker doesn't particularly enjoy dancing. "How come?" is slang meaning "Why?"

A native English speaker may say..... "It's not my cup of tea"is used to refer to something that he or she does not like. "Rock Music is not my cup of tea" How come may also be used by a native English speaker either in American, Hiberno, or British English. "How come" is used as a question. "How come you are learning English"? (What is the reason that you are learning English?)

"cup of tea" is an idiom. Example: "Dancing is not my cup of tea, I prefer singing." ”How come“ means "how could it happen?". Example: "How come you missed the stop again?"