While there are certainly many passionate people on both sides of the gun control debate, gun owners, as a whole, tend to be more passionate about the issue than do gun control proponents. This is particularly true in light of the number gun-owning Democrats, some of whom might swing to the Republican column if their legislator were to vote for a gun control law they didn't like. Legislators know that, and it matters, because they don't want to lose the gun owners' votes.
Additionally, there is the 2nd Amendment.
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.
The meaning of that is unfortunately rather unclear, and this fact has led to much debate. However, in 2008, the Supreme Court held in a 5-4 vote that the right to bear arms is an individual right. As with any 5-4 ruling, one has got to reckon that it could be overturned some day. But for now, it is the law of the land.
It should be added that the Court has allowed various restrictions on gun ownership, such as mental health restrictions, criminal background restrictions, and restrictions against particularly powerful weapons. So a gun control lobby can do certain things, but if they try for too much, they will run into the 2nd Amendment limit.
EDIT: In regards to the level of passion:
Obviously, it can't be measured directly. But there are plenty of proxies. A person who believes something, but not very passionately, would be more likely to answer a poll in accordance with their belief, but might be less likely to take interest in a single-issue organization, magazine, or website. On the pro-gun side, a simple google search took me straight to [guns.com], which, as of right now, has eight different stories on the front page, all of them written today. Clearly, the people in charge of the site are putting a lot of effort into it. If they weren't, some of the stories would be older. Then there is of course the NRA, Gun World Magazine, Guns&Ammo, Firearms News, Guns Magazine, with many more linked from those in turn. It's the same thing on youtube; here is someone's review of 10 gun channels, for example.
On the gun-control side, there are also single-issue groups, such as Brady. But a lot of the action seems to come from general-interest left-of-center organizations, such as moveon and Democracy Now. It's similar on Youtube; here is a pro-gun-control video from Al Jazeera; here is another from The View. These are people for whom gun control is just one issue among many.
The passion difference is so pronounced that even the New Yorker writes
. . . The N.R.A.’s biggest asset isn’t cash but the devotion of its members. Adam Winkler, a law professor at U.C.L.A. and the author of the 2011 book “Gunfight,” told me, “N.R.A. members are politically engaged and politically active. They call and write elected officials, they show up to vote, and they vote based on the gun issue.” In one revealing study, people who were in favor of permits for gun owners described themselves as more invested in the issue than gun-rights supporters did. Yet people in the latter group were four times as likely to have donated money and written a politician about the issue.
and here are similar write-ups about the "Enthusiasm Gap" from the BBC and The Atlantic. Here in the Washington Post, gun control advocates are claiming they have closed the enthusiasm gap, which obviously implies that it existed in the first place. It's also interesting that the gun-control people are merely claiming equal enthusiasm, in relation to an event that worked out in their favor.