It's also possible they were competent but don't have the required staff to handle a large protest. Under normal circumstances, they might request help from other groups (DC police, national guard, whatever) but due to jurisdictional restrictions help can't come unless it's approved at high levels and no approvals were given.

In other words, they may have been set up to fail.

(There's still the issue of that video of protesters being let in, which would imply that capital police do have some explaining to do.)