When you boil it all down, this is the answer. The main reasoning behind no knock warrants is to prevent evidence destruction. The harm of a drug dealer flushing some drugs down the toilet versus potentially killing innocent people isn't even in the same ball park. And really, how many flushes can some get in the 15-30 seconds that it takes for cops to rush into the house?
Sometimes the argument is that no knocks are for armed and violent suspects but even then it's not clear a no-knock raid makes it any safer for anybody. Police could just as easily wait until someone leaves the house in the morning and execute the arrest with far less risk.
Seems like no-knock raids should be outright banned for local law enforcement at the least.