From a Sky News article:
Previously in the US, waiting for specially trained tactical teams was the standard response to "active shooter" incidents.
But this changed after the 1999 Columbine High School shooting in Colorado, when 12 students and a teacher were killed while police waited an hour for a SWAT team to arrive.
Responders usually operate in teams of four, so the guidance changed to encourage the first four to pursue the suspect.
This has since been changed further to emphasize that police officers should do all in their power to stop the suspect shooting and taking any more lives.
The Advanced Law Enforcement Rapid Response Training Center (ALERRT) at Texas State University is the largest active shooter response training provider in the US. They are recognised by the FBI and are responsible for training police across Texas.
Their guidance states: "Law enforcement's purpose is to stop the active shooter as soon as possible. Officers will proceed directly to the area in which the last shots were heard."
It stresses that the "first priority is to move in… bypassing wounded and confronting the shooter".
Who knows why this guidance wasn't followed. There doesn't seem to be any indication that race was a factor. Maybe fear or lack of training.