Because Assad is pure evil. And, Iran and Russia willingly look the other way or support this kind of barbaric terrorism.
Even Trump is now seeing how suddenly the Presidency can change your entire worldview, and impose a sudden and great sense of responsibility that overwhelms any previously stated belief or goal.
Obamacare didn't undermine Obama's second term. Neither did birthers, scandal, or a very poor relationship with the GOP. What destroyed Obama was failing to act in a time of great moral injustice -- idly standing by and vacillating over his own red-line.
I pray that Trump doesn't make the same mistake. He's yet to show much moral courage or understanding of the gravity of his new job. He was one of loudest voices urging Obama to embrace cowardice, and that the US should shirk its moral obligations. Perhaps all this changes with Syria. Assad must go - - and he must go soon. Who cares if Russia or Iran objects? Since when did the USA give a damn about their opinions or policy goals?
I agree. Sometimes we must act regardless of the consequences. This is one of those times.