David Karol provides some answers:
[G]un control supporters have no shared social activities, no common identity and no companies that cater to them. Their jobs don’t bring them together. Unlike gun rights advocates’ they don’t find and stay in touch with each other without a conscious and sustained effort to do so. Under these conditions, it is not surprising to find far more effective mobilization of sentiment on the gun rights side. So even if there was significant intensity of feeling on the part of a sizable minority of gun control advocates, (say 10% of the 90% favoring background checks) we should expect them to have greater difficulty in channeling those feelings and building durable political organizations.
Chait explains why Senate Democrats folded:
If you’re picking your battles, background checks are as good an issue as any to lay down. For one thing, as I’ve suggested, guns loom disproportionately large in the political world of red state Democrats. Guns are the way they signal home state cultural affinity, giving themselves a chance to get their economic message heard. Their A rating from the National Rifle Association is powerful shorthand. And yes, the NRA is crazy and partisan, and was opposing a bill it used to support and that most Republicans support. But none of those facts overcomes the blunt reality of the A rating’s political value.
What’s more, this particular gun vote was an especially good time for Democrats to defect. None of them cast the deciding vote; it fell six votes shy of defeating a filibuster. The bill was already a compromise of a compromise, something that would have stopped a tiny fraction of gun crimes. Even if it passed the Senate, it faced steep odds of passing the House, where it probably would have died, been weakened further, or even turned into a law that weakened existing gun laws.