I think there are in general 2 arguments. Firstly, there have been multiple studies linking cannabis use to schizophrenia, and opponents like to point to them.

I think there are in general 2 arguments. Firstly, there have been multiple studies linking cannabis use to schizophrenia, and opponents like to point to them. But most of these studies are of poor quality (e.g. small sample size, short duration, poorly controlled for confounding factors), or/and have skullduggerous sources of funding. Secondly, decades of anti-drug propaganda have much of the population convinced that all drugs (except of course alcohol, caffeine and nicotine) are essentially evil, and that cannabis in particular is a "gateway drug" to things like heroin (personally I find this to be ludicrous, but it's been said enough times by tabloids, politicians and schools that enough people believe it). This second one is deeply entrenched, and it will take much time to change opinions.