Those rioters were symbolically protected by Trump via his encouragement and endorsement of that "protest" turned insurrection/riot.
In that sense, I can see where a lax attitude toward security was taken seeing as if it had a stamp of approval from the President of the United States himself. This along with the attitude and understanding of the rioters that they had the express permission and encouragement of their president to interpret and act on his approval in any way they saw fit of which in large part was to support their president by relieving their angst by "patriotically" taking over the Capitol in an overwhelming show of force.
As well, there were opportunists within the mob who had their own agendas of which had nothing to do with supporting their president. Herd mentality is also a fickle thing and starting a stampede doesn't take much encouragement of which I think also contributed to the chaos at the Capitol.