Why did Michael wait 21 days to come to help the angel that was sent to Daniel?

In Daniel 10:2-3 ,12-14 (NIV) we read: ²At that time I, Daniel, mourned for three weeks. ³I ate no choice food; no meat or wine touched my lips; and I used no lotions at all until the three weeks were over. ¹²Then he continued, “Do not …