✔ 最佳答案
Chiaki said the fact future people can travel through time can't be known by present people. Since Makoto already knew that, so he can't keep in touch with her. I think that's a rule of future. That's why he had to leave. When he used his last time leap, everyone went back to the past, including Makoto. And in the past, Makoto still had 1 time leap. Therefore, she jumped back and gave that time-machine to Chiaki, so he can go back to future. What he said to Makoto before his leaving was actually "I'd be waiting for you in the future."