Because people are squeamish about chemical weapons. Remember, President Obama (not a hawkish president by any stretch) said this

We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation

Now, it turns out it was an empty threat, but it's noteworthy that he felt that he needed to do this over chemical weapons. As has been noted, this doesn't affect the ability of Assad to kill people with other means

Look, I get why — morally, strategically, and legally — chemical weapons are different than conventional ones. But if my entire family and village were wiped out with bullets and bombs rather than chemical weapons, I wouldn’t draw much solace from any of these distinctions.

As to what we got out of it? Public Relations

The difference, as Trump admirably admitted from the Rose Garden, is that he’s president now and that changes your perspective on things. It’s always easy to throw brick-bats when you have no responsibility. Now he’s looking at the prospect of being the president who, in effect, sanctioned the use of chemical weapons, a violation of international law. As he put it in his statement Thursday night:

It is in this vital national-security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.

There can be no dispute that Syria used banned chemical weapons, violated its obligations under the Chemical Weapons Convention, and ignored the urging of the U.N. Security Council.

That is a sound argument. But it was just as sound in 2013 [when Trump vowed not to take any action in Syria]. Trump’s real motivation seems to be the fact that babies were “choked out” and that he saw it on TV. And it is this apparent fact that should give everyone — supporters and critics alike — the most cause for concern.

Ultimately this was an easy win. Trump got

1. Bipartisan support (many Democrats praised the effort, which was an easy win for them too)
2. No larger military commitments
3. Avoided any serious entanglements with Russia (whom we warned about the strike)
4. Didn't really do any serious damage
5. Sent a message

Because people are squeamish about chemical weapons. Remember, President Obama (not a hawkish president by any stretch) said this

We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation

Now, it turns out it was an empty threat, but it's noteworthy that he felt that he needed to do this over chemical weapons. As has been noted, this doesn't affect the ability of Assad to kill people with other means

Look, I get why — morally, strategically, and legally — chemical weapons are different than conventional ones. But if my entire family and village were wiped out with bullets and bombs rather than chemical weapons, I wouldn’t draw much solace from any of these distinctions.

As to what we got out of it? Public Relations

The difference, as Trump admirably admitted from the Rose Garden, is that he’s president now and that changes your perspective on things. It’s always easy to throw brick-bats when you have no responsibility. Now he’s looking at the prospect of being the president who, in effect, sanctioned the use of chemical weapons, a violation of international law. As he put it in his statement Thursday night:

It is in this vital national-security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.

There can be no dispute that Syria used banned chemical weapons, violated its obligations under the Chemical Weapons Convention, and ignored the urging of the U.N. Security Council.

That is a sound argument. But it was just as sound in 2013 [when Trump decried action in Syria]. Trump’s real motivation seems to be the fact that babies were “choked out” and that he saw it on TV. And it is this apparent fact that should give everyone — supporters and critics alike — the most cause for concern.

Ultimately this was an easy win. Trump got

1. Bipartisan support (many Democrats praised the effort, which was an easy win for them too)
2. No larger military commitments
3. Avoided any serious entanglements with Russia (whom we warned about the strike)
4. Didn't really do any serious damage
5. "Sent a message" about chemical weapons