The problem is the queues are actually making it far easier for the terrorist. He/she doesn't care where they kill people, just that they do, and the risk of getting a device into a stadium, even without security, is far greater than just pitching up outside, and hitting a queue. To get inside they have to purchase a ticket for starters, which with 'watched' people might flag an issue, whereas you can jump on a bus in Ashton, get off outside wander across the road, and just stand near a queue.
If not a suicide type attack, it would also be very difficult to get say an automatic weapon into a stadium, whereas you can drive by in a car with the window down, and just shoot at random.
Its pretty much impossible to drive a lorry into a stadium, whereas a queue on the pavement.....
Queue's are at bigger risk than people inside a stadium in my opinion.
I'm far from convinced this is anything to do with security, more of a a side show,
@jrb has posted his conversations with the police and stewards earlier in the thread, which suggest its not at the request of the police. Its causing huge inconvenience (especially at night games when people invariably will turn up later due to work), and its something that can be easily rectified by putting the correct resources (searching staff) in place outside, to make sure the queue's don't form in the first place.
The only answer City give is "turn up earlier", whilst they themselves, continue to entertain at City Square until 15 minutes before kick off, why ?