The same reason we do not crack down on our own citizens killing each other (or innocents) in the streets? The same reason we're slow to act when children are gunned down in school shootings?

I wish I had a solution. I don't.

Whether I call my congressional leader to say support or don't support Syria, nothing is going to change. Humanity has to change. Aside from being friendly, peaceful to whomever I may encounter, I kind of feel like everything else is really out of my control. So, as I said to someone I recently encountered who did not speak English well (and considering my Spanish is poor), "A smile says a lot."

There are a couple differences between machine guns and chemical weapons.

Chemical weapons can very effectively murder masses of people, all while the people using them can be out of harms way with gas masks and such. And they are cheap.

The manner of death is extremely painful and torturous. Reportedly they are using Sarin Gas in Syria - "Like other nerve agents, sarin attacks the nervous system. It stops nerve endings in muscles from switching off. Death will usually occur as a result of asphyxia due to the inability of the muscles involved in breathing to function"

Chemicals can also effect a wider range of people and do so over an extended period of time. Some chemical weapons cause cancer, which may not be apparent for many many years. Yes, machine guns can injure without killing as well, but don't have the same lasting effects as chemical weapons do. Because of this chemical weapons are considered more of a torture device than tradition guns/rockets/ammo are.

Conventional weapons, like rockets and machine guns, are still traditionally thought of as military tools. They can kill civilians as well, but the long standing ways of war (which are slowly changing) place them in a category of military vs military, not military vs civilians. The chemical warfare that we are seeing in Syria is military vs civilians and shows extreme callousness, and seen as something that should be dealt with immediately. The fact that they are openly targeting civilians with chemical weapons is not a problem between political or military opponents, but is wiping out civilians. This opens a bunch of doors because traditionally wars should be to sort out issues, resolve matters, etc etc. The civilians should stay out of harms way as much as possible so that once a resolution arises, they are able to be alive and healthy to kickstart the economy, return to daily life etc. While there are always civilian casualties in wars, the nature of chemical weapons changes the world's view and outlook on what is being accomplished in Syria.

A bit of a messy slew of thoughts, hope this helps explain a bit. I'm not an advocate for any type of violence, chemical or machine guns, but this is why the media and political shitstorm is happening over the chemicals rather than the machine guns.