Despite topping the world with incarcerations and arrests and law enforcement funding, the US is not a particularly safe place, so obviously US law enforcement isn't focused on safety and justice, which leaves the monetary factor.

If you arrest someone for a drug crime and get a plea deal out of them they get jail fees, processing fees, court minimum fees plus any additional court costs, probation costs and fees, multiple "X state specialty tax fee", plus kickbacks from the mandatory court ordered drug/anger/traffic class, cost of drug test fees, etc. If you arrest someone from murder and imprison them for life, sure you can claim to charge them those fees, but they will never be free of prison to ever pay them. So it should be no surprise that cops are incentivized to go for easy drug charges from non-dangerous citizens that puts money in their pockets over actual dangerous criminals who will only reduce department revenue.