https://theconversation.com/was-it-a-coup-no-but-siege-on-us...
In short, a coup is an organised action to seize power. For that, it's essential to have secured the support of at least part of the armed forces, and a plan to replace the top of the power hierarchy. Nothing like that was seen in the Capitol riots, which seemed disorganised and essentially demonstrative in nature. Once entering the building, the rioters proceeded taking selfies.