Why didn't you say so?

What does “Why didn't you say so” mean?

You should have told me that earlier. Said when someone finally mentions something important.

• Till death do us part./tɪl deθ duː ʌs pɑːrt/
• Is that all you got?/ɪz ðæt ɔːl juː ɡɑːt/
• We go way back./wiː ɡoʊ weɪ bæk/
• You have the floor./juː hæv ðə flɔːr/
• Are you insane?/ɑːr juː ɪnˈseɪn/
• What's the catch?/wʌts ðə kætʃ/
• Low key./ˌloʊ ˈkiː/
• You are impossible./juː ɑːr ɪmˈpɑːsəbl/