They actually did incorporate accidents, remember in "The Thespian Catalyst" they faked Howard's death comically, and also made the fact Raj wanted him to die comical.
In sitcoms, grief is usually turned into shadenfreude. Given the position and strength of the show, it would be compromising it to make Sheldon grieve.
Many people have mentioned his meemaw dying - I don't think they could do that comically in any way (like they did with Charlie's death in Two and a Half Men). Remember that in first Will Wheaton episode he says "I can't imagine a world without my meemaw." and let's himself be