Mostly because the first amendment doesn't impose a qualifier. "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances." I can't see anything that would limit freedom of speech, press, assembly, religion or petitioning. At all. I don't see how any of that is open to interpretation.
However, when the second amendment has a qualifier to it that is VERY much open to interpretation. "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed." It's not as clear as the first amendment. You could easily argue that if my right to bear arms can't be infringed, I should have the right to assemble chemical weapons or purchase a full-auto gun with incendiary rounds. You could also interpret it to say that you could only own firearms. The language was made in the days of muskets and flintlocks, and it's a little bit dated considering the incredible scope of items people have made to murder each other with since then. That's why there's a discrepancy. For me, it has nothing to do with a dislike of guns or "hating America"(copyright Fox News), but has much more to do with wanting weapons that have no place in a civilian neighborhood (and I really don't see how anyone can argue that a civilian homestead needs armor-piercing rounds or an assault weapon). If someone wants something like a revolver, a rifle, shotgun or anything else along those lines, they can go for it. I just have issues with military-grade weapons in the hands of civilians, and I think that's a fairly reasonable thing to take issue with.