No they didn't storm the Capitol, they were let in. There is video of police waving people in.
In order to get into the Capitol you have to have a time-stamped ticket requested months in advance. The only way people could get in without a ticket would be if someone waved them in, which the Capitol police did.
Why did the police let them in? I am sure the natural idea for the Trump supporters was to go near the Capitol Building, and when the police wave them in and when a door is open what is anyone supposed to think?
Some people - maybe a couple of hundred - fought with police, overpowered them, and DID storm the Capitol. Later on, others were let in.