It's not that easy. We had similar with the Manchester arena bombing. "Why didn't the emergency services react faster?", "Why didn't the police stop the bomber before he was able to kill people?". They follow process, but until the event happens, you don't know if the process is watertight.Imagine they stood around and waited for an hour wrestling parents to the ground who just wanted to try to save their kids whilst an active shooter murdered 19 of them. Since Columbine the rules of engagement for a school shooting is for the police to get in there as fast as possible. Apparently Texas didn't get the memo.
Then you factor in the people element. You don't know how you'd react in that situation.
Blaming the police reaction doesn't solve the problem. If the police had stormed the building and killed the gunman after he'd only killed 10 kids, would that be ok? What if they'd stormed the building and accidently shot a teacher, or the gunman killed more people (including police officers) in reaction?
The only question that really needs answering is why an 18 year old was able to buy a number of automatic weapons and a significant amount of ammunition with no real effort or checkpoints?