I think the main problem is the big banks are right next door to the Silicon Roundabout area, and they typically pay 3 times as much.
Also side rant. To all non-londerners, Tech City / Silicon Roundabout is an abstract concept that doesn't actually exist in any concrete form. Unless someone told you, you wouldn't even notice you were passing through it. Infact the only remotely "tech looking" building is satellite company Immarsat's HQ office, and IIRC they were around long before "tech city" had been dreamed up.