Raptor Politics:

raptorpolitics.org.uk/2018/01/24/five-years-of-failure-rspb-withdraws-from-peak-district-bird-of-prey-initiative

raptorpolitics.org.uk/2018/01/23/hen-harriers-peregrine-are-likely-to-disappear-as-a-breeding-species-from-english-red-grouse-moors-within-the-next-decade-or-less

I'm guessing, but it looks to me that the RSPB felt they had to try a soft collaborative approach first; I wouldn't be surprised if they now resort to harder, more expensive legal challenges & ask difficult questions of the police about the prosecution cases that failed. I can't see them dropping it - it's high profile & it's their mandate

I'd recommend reading the raptor persecution UK website for more detailed background. I can fully understand why they have pulled out given one of the key partners seems to be unable to admit that persecution by game shooting (grouse) interests is one one of the main problems.