This is a really big question, but I'll try my best at hitting all the bases.

So first of all, the "mainstream left" are liberals, which are distinctly not leftist. Liberal ideology is pro-capitalist, so working class and poor people with too much power is a problem for the liberal establishment. That said, it's also a problem for mainstream conservatives, but their interests are slightly different.

Both parties are parties of the rich and powerful, but to rule, they need to manufacture consent from the rest of the population. So, both are in the business of convincing people to vote against their best interest through rhetorical games and propaganda. The main demographic for liberals (simplifying) are academics, large unions, and people of color. Academics tend not to be very pro-gun, big unions are more tools of control of the worker-class than actual bastions of worker-power, and since we live in a white supremacist society, giving guns to people of color is something the gov't has no interest in doing (see the history of the Black Panther Party as an example).

On the other hand, the Conservative's main demographics are small business owners, religious Christians, farmers, etc. This demographic tends to be overwhelmingly white and property owning. Also, weapons manufacturers are big donors to the Republican Party. Thus, conservatives are in favor of guns, but construct said laws in a way that makes it far more difficult for poor people of color to obtain them legally. So, really, it has very little to do with supporting gun-rights, so much as making sure the people they like are armed and the people they don't like aren't.

Really, when it comes down to it, both parties will usually support giving more control to police and stiffer punishment for people who take the law into their own hands. They're goal is to maintain the status quo.

Socialists (and communists, and anarchists) are explicitly against the status quo. So, to us, arming the poor, people of color, and other oppressed groups is a step toward breaking the status quo. It has very little to do with the 2nd Amendment for us, though it is rather convenient (except for those of us that live in cities that basically make legal gun ownership impossible).

So, I tried my best at explaining it. I probably missed something or have holes in my argument, but I hope I did at least a half-decent job. Also, judging by your username, you might wanna head over to /r/anarchy101 and ask some questions about anarchism. A lot of anti-gov't sentiment tends toward the right-wing in the US, because I think a lot of people in the US associate the left-wing with gov't control. Anarchism is a left-wing, anti-state political philosophy that might interest you.