The stadium is in the city centre. It is in a very pleasant part of the city centre, surrounded by a warren of old, narrow streets which have been tarted up and colonised by trendy restaurants and pubs. It is a bit like Covent Garden, so imagine the carnage which would be caused if 70,000 football fans poured out of a ground into Covent Garden. Wembley, for all its hideous faults, has the distinct advantage of being in the middle of a scruffy wasteland. There are a couple of Indian restaurants, a chippie, a greasy spoon cafe and a new McDonald's, but very little to make you tarry after a match.
Most people head for the underground or drive off towards the North Circular as quickly as possible.
Sometimes punters coming out of Wembley trek off towards Trafalgar Square, or some other rendezvous in the West End, and sometimes there is trouble there. But there is seldom any bother around Wembley because everybody is in a hurry to go somewhere nicer. In Cardiff, in contrast, fans will spill out of the stadium and straight into the very pleasant streets full of inviting pubs and bars. The police will doubtless try to keep rival fans apart and encourage them to go home but unless they cordon off the whole city centre and station water cannon on every corner the area will become unpoliceable. There are additional, inflammatory factors.