The partial closure of parts of the criminal justice system during COVID led to a backlog which in turn appears to have pushed the system to a point it has been unable to properly recover from - there are some very interesting statistics here [0].

As to why that is - the explanations I've seen generally feature incompetence amongst various parts of the system and a degree of underfunding (or perhaps poorly managed funding) - including the fact that there is a shortage of criminal barristers due to poor pay. Juries themselves don't seem to be cited as a huge problem.

0. https://www.gov.uk/government/collections/criminal-court-sta...