It's pretty damn simple why people care about the death of celebrities, especially universally beloved celebrities like Robin Williams.
If a family member or friend of yours died that would be tragic, right? Of course, to me, their death would have very little impact. Simply put, I didn't know them. I can feel empathy for you and them, but I personally won't be upset. I don't know them.
But I know Robin Williams. I grew up watching his movies, interviews, and stand up routines. He has made me laugh, smile, and even moved me for my entire life. My parents know him and remember him from shows that I am too young for. He touched the lives of millions of people.
That's the difference here. It affects more people as more people knew him.
What's happening in Gaza (honestly, I'm way more concerned with ISIS as they are 100 times worse than whichever group you hate in that situation) is bad, but not nearly as personal. Very few people in the West personally know people there. It's much more distant.
Besides, no one said I can't be concerned about more than one thing at a time.