This is down to Landlords being unrealistic, a lack of "incentivisation" by Local Authorities, who won't keep on top of the condition of properties and use enforcement action and I think six months of vacancy before the landlord has to start footing the bill for Business Rates is a little too long- although it is a great incentive with the more dilligent landlord who realises that filling a property is better than leaving it empty.
However, the high street as we knew it is in it's knees and retail has changed forever. Have to remember that it really started way back with the Supermarket and Retail Parks and the internet is simply the last nail slowly being hammered home....