The reason is that a bacterial plant disease that can affect hundreds of different trees and herbaceous plants has struck several European countries. It seriously damaged olive trees in Italy.
The disease is spread by sucking insects that move the bacteria from plant to plant. The disease has not been found in the UK, but they are concerned that it could get there. Scientists want to know where insects live that could possibly carry the disease in the future.
Apparently the disease can be spread by sucking insects in general, not just spittlebugs in particular. They could include leafhoppers, planthoppers, plant bugs and more. For that reason, I am guessing that they want to survey spittlebugs rather than other sucking insects simply because they are easy for the ordinary person with no training to see.