After talking to a couple of WGs during punts I usually ask this myself, and common answer seems to be that they can't pull in the rates they're used to elsewhere in the country, i.e. touring here punters would expect to pay £120 or less p/h whereas the WGs can easily demand £150 p/h elsewhere, so it's just not worth their time to come here because they can easily get booked up if they just tour other areas.
Think the low expectation of price here is because of EEs lowballing and getting more of the business that is available.