...why didn't the police and national guard show up on time, and in a way that was coordinated?
What do you mean by the term 'the police'?
There are 105 federal law enforcement agencies.
Not all 105 federal law enforcement agencies have anything to do with DC, but the District of Columbia has a large number of different law enforcement and security jurisdictions all the same. Control of the various agencies with a direct role in DC does not lie in the same hands. It is very possible that communication among them isn't what it should be where the District of Columbia is concerned.
There is the District itself, the City, whose mayor is currently Muriel Bowser. DC has its own police, who serve and protect its citizens and also patrol marches, etc.
There is the Department of the Interior and its police. The D of I runs the many national monuments and provides security for them.
The Capitol Hill police fall under the remit of Congress. Their sole duty is the protection of the Houses of Congress, Congressional parks and thoroughfares in DC and in some other locations.
There is the Dept of Homeland Security and also the FBI.