Regardless of how & why the tragedy occurred, the fact is that we do not have an infinite budget to spend and infinite time to work on any particular case. There is inevitably a point at which, if it has not been solved it must become low priority, effectively waiting for something new to come up, while more recent, more solvable cases are progressed.
I'm sure there are other cases that have not had a fraction of the attention this one has had.
Sad it may be but there is always a point at which a UK citizen has to be effectively given up on, save for any future new lead.