The problem is that the ash cloud is now very widespread across Europe. They know the affected areas but within them is it hard to classify ash densities from area to area. With high altitude winds, the situation changes almost hourly and it is simply too difficult for anyone to put their job, and thousands of lives at risk by opening up a section of airspace saying "It should be ok!".
Having said that, a couple of transatlantic flights have arrived in Europe within the last few hours.