Mainly because the morning time is 9: 00- 10: 15.

It is precisely because of this 15 minute break that the K-line trend of the whole market is defined.

Taking the K-line of 15 minutes as the benchmark is the coincidence of the daily line, and other larger periods have obvious fractures relative to the daily line.

Of course, there is no fixed law, and all trading methods and technologies vary from person to person.