I feel like this question is based on a lack of understanding of what people mean when they say legalizing drugs reduces crime.
Legalizing drugs reduces the crime of drug possession simply because it is not a crime anymore. In exactly the same way, legalizing carrying a gun without a license reduces the crimes associated with carrying. That said, it does not necessarily reduce crime overall, especially without an increase in support for rehab and addiction services.
Portland, Oregon reduced the severity of possession crimes from a felony to a misdemeanor for schedule 1 and 2 drugs, and the result was a decrease in felonies, but an increase in misdemeanors proportional to the felony decrease, suggesting that overall arrests remained the same. While there was not a significant increase in crime overall, the growing popularity of fentanyl during that time did lead to an increase in deaths, and public opinion seemed to indicate that there was a change in the perception of public safety during that time even though the crime rate remained mostly stable. In my opinion, I think it probably had to do with an increase in the visible public consumption of drugs and the unpleasant realities that can come with that.
A primary difference between drugs and guns is that drugs more often lead to the death or injury of the user while guns are more likely than drugs to lead to the death or injury of someone else. Furthermore, a gun enables the commission of a crime in that you can’t rob a liquor store with a joint, but you definitely can with a firearm.
That said, I live in an extremely gun friendly area and have carried both openly and concealed, yet the perception of “how safe is my neighborhood” seems to be swayed much more by the growing homeless/drug using population than the sight of a person with a gun on their hip does. I have never seen someone avert their gaze and pick up their pace for an open carrying person around here, but I see it all the time when they encounter a drug using homeless person. I know that is mostly anecdotal, but when paired with the data from Portland, I think it can be safely suggested that the perception of public safety has more to do with sights that make people uncomfortable over actual statistical crime data.
It is very possible to live in a place that is statistically safer, yet feels more dangerous, entirely due to the discomfort caused by witnessing human suffering, which is less of a factor when witnessing someone carrying a gun. Furthermore, decriminalizing drugs in one particular area can lead to an increase in drug users inhabiting that area, thus reducing the perception of public safety regardless of crime statistics.
Legalization of carrying a gun primarily leads to an increase in concealed carry, thus hiding the increased presence of guns from public view, and therefore leaving the perception of public safety unaltered. Concealed carry is more popular than open carry because it increases the tactical effectiveness of the firearm by not making you an obvious target of a criminal, so it is the preferred choice for most gun carriers, both legal and illegal. Contrastingly, it seems pretty hard to hide the fact that you’re totally zooted on fentanyl or methamphetamine.
Overall, I’d say that simple decriminalization does not have a significant effect on crime. Most drug users are not homeless or violent criminals, so they simply fly under the radar just like a concealed firearm carrier does. However, an increase in public intoxication or public discharge of a firearm does decrease the public perception of safety, regardless of actual statistical consequences, and politicians create a narrative based on that public opinion rather than using the true data about public harm.