Because:
There's been countless investigations that have found Hillary not responsible for the Benghazi attacks that the RNC was trying to paint, including investigations run by the Republican party.
The Democrats were going after the RNC for playing with her emotions and continuing to push a false narrative. Trump (not the Republicans) was going directly after the Khans in a personal way.
Pat Smith was attacking Hillary over something that just wasn't true. She was throwing the incorrect Benghazi thing out there.
The Khan family was attacking Trump over his xenophobia and bigoted ideas, while they were up there to talk as a family of a fallen soldier who is regularly offended over Trump's remarks against their people (Muslims and minorities) who have fought/died for the country.
Pat smith was going after an incorrect blame.
The Khan family was going after rhetoric.
The Democrats were more disgusted that the RNC would play with the poor woman's emotions and fuel her incorrect emotional belief of who is to blame. Hillary said nothing.
The Republicans said not much about the Khan family's speech. Trump took the bait and attacked them personally.
Stop trying to justify Trump's actions and handling of this. Not only was he out of line in his first attack, but he doubled down... and tripled down.
These aren't the same.