Critics of legalization think that expanding the pool of drug users is a recipe for disaster. Criminalizing drugs has kept people from becoming users. It makes more sense to address the current issues than to create another mess.
However, decriminalizing drugs brings drug use out of the shadows so that addicts can get the help they need. People are dying because they are using unregulated drugs in unsafe ways. People are obviously not going to stop using drugs. Instead, harm reduction methods will help save lives in the short term, and can help people get off of drugs entirely in the long term.
Drug abuse is a public health issue. Crime becomes a factor because of its prohibition—gangs would not be in control of the drug market if it were legalized. Portugal legalized drugs in 2001 to great success.
Critics argue while that strategy worked in Portugal, the United States is infinitely more complicated. It's not enough to change the laws, but cultural perspectives on addiction need to change. From The New Yorker:
"... but what works in a small seaside nation in Southern Europe is not necessarily likely to work in a place that is larger, richer, and more heavily plagued by drug abuse."