onyx - as a very general guess, comparing a regional airport in Germany with an island airport in Greece is not an equal comparison. One airport is a major exporter of people going away for 2 weeks in the sun, while the other imports people visiting for 2 weeks of sunshine. Despite the efforts of the tourist board, Germany remains a significant net spender on tourism rather than being a recipient of funds.
The average Greek island relies on good weather and depends very heavily on summer tourists to make enough money to cover the winter when there is little work and few jobs. The average Greek beach in January just isn't that appealing.
The average regional German airport, e.g. Bremen or Leipzig, is close to a city whose economy is active throughout the year and predominantly weather independent, much like the economy of Sheffield or Doncaster.
For DSA to be credible and demonstrate that it's useful to the local and UK economy and not just a way of helping people spend their money in a different country, there should be some non-beach flights (e.g. Wizzair) all year round