Not least because the council vehicles and buses all routinely ignore the limit so have disproportionately high closing speeds and of course are not what pedestrians expect.
I frequently use a main access road into the town centre that was reduced from 30 to 20 several years ago as part of a fair sized area 20 mph scheme mainly of narrow Victorian streets.
So far in several years useage since the change, I have regularly followed police vehicles, ambulances, city council vehicles, service buses, and perhaps most of all cyclists breaking the limit. None of the emergency vehicle drivers used blues & twos, so weren't apparently on anything other than routine runs. As far as I can see only three groups consistently obey the limit: tourists and other out of towners, taxis, and local residents coming off side roads. I understand the taxi drivers disagree with the limit, but a speeding conviction is disastrous for their license and insurance.
Despite signs saying 'Police Speed Check Area', I've never heard of the police enforcing the limit and the only checks are occasional groups of local residents in yellow high-vis jackets who have no legal powers using a hand-held speed gun.
20 mph is appropriate for the majority of the area being narrow streets of Victorian workers terraces with cars parked on both sides. However, speeding wasn't generally a problem in them as the usable space between the parked vehicles isn't usually wide enough to allow two cars travelling in opposite directions to pass and barely allows buses and council waste lorries to get through. (Oftentimes they can't at that!) In fact, IIRC the bus diversion through the middle of the area was firstly rerouted to be one-way so two buses couldn't meet head-on, then the bus company removed the loop run into the residential area completely thanks to the frequent delays and stoppages. So nobody other than those on two-wheelers can drive at over 20 anyway and motorists didn't need signs every few hundred yards to tell them so. This is one of five or six such schemes in the city I live nearby and the local newspaper cited the costs at £500,000-600,000 for each.
