It could be incompetence, it could be lack of ressources, it could be that it’s just very hard to find the culprits and prove with high certainty they are guilty.
Also, excessive focus on proportions of crimes solved results in the worst sort of policing, where they push clearance rates by trying to pin lots of charges on one weak individual, decline to record minor crimes that probably won't be solved, and boost their overall clearance rates with easy charges for drug possession.