I'm in Europe. I could get weed within 3 minutes, on the street. Hard drugs would maybe take a day. But I have no idea where I could buy an illegal gun. Not saying it isn't possible, but it seems to take a bit of work.

That sort of friction doesn't deter a would-be bank robber, but by and large the gangs here don't acquire guns. I've seen a few ugly scenes in public transport, and I'd be hesitant to use it if I thought the people who have knives today had guns.

So that's the answer to your question. It's the same as "why are shoulder-launched AA missiles illegal in the US – only criminals will use them".

The other part of an answer is that as far as I can see the deaths from accidents and suicides are vastly more important, and I believe even for crime, it's still an open question if gun-owners are safer.

Obviously all this depends on the low amount of firearms in circulation, so it doesn't directly apply to the US. There, it would require a transition period with buybacks etc. – which may or may not have worked quite well in Australia.