It's possible that they didn't expect the sort of riotous response they got but I also think the lack of security forces was by design — they were never meant to be able to stand up to those that Trump had told to "stand back and stand by!" I say this because of the way the National Guard was kept at bay and not allowed to enter the fray to try and stop it. Trump was truly hoping his terrorists would succeed. 

I also think it's important for those who will sit in judgment of what played out to be mindful of the fact that Trump had numerous advisors who were giving him information in real-time as to what was happening in the days and weeks leading up to the insurrection and in the moment as it happened. He can't claim any sort of ignorance. He was fully aware and should have stepped in and used his position of authority to put a stop to it. 

Kevin g