How did one meteor kill whole bunch of dinosaurs?

First off, while we know that the asteroid was the deciding factor, it's very likely that it wasn't the only cause. The number of dinosaur species in the world was shrinking before the impact, and the volcanic activity and changing climate was at least making things very miserable for them. It's more likely that this K-Pg event was just the final nail in the coffin, as it were. Even so, it was a heck of a nail, and it's very important to remember:

1. The asteroid that struck the earth was five miles across- as big as Mount Everest.
2. The place where it struck, the Gulf of Mexico, happens to have a layer of limestone under it, which provides quite a bit of debris for at least the next three thousand miles to worry about.
3. The original clue we received that it was an asteroid- an inch-thick layer of iridium- was found around the globe, meaning that fallout stretched everywhere.
4. An impact of that magnitude would have disturbed the plate tectonics to some degree, leading to a higher surge in volcanic activity, earthquakes, and all of the anomalies that come with those.
5. The amount of dust and ash sent into the air would have blocked out the sun, preventing its warmth from reaching the surface and its light from cycling photosynthesis in plants. The world would settle into a 'nuclear winter' of sorts, becoming a deep tundra over the several generations that it would have lasted.

Fortunately, the last point brings up a good idea of why what survived did. Dinosaurs, over the course of their 180-million-year hierarchy, began to grow specialized, being only able to live in specific environments and eat certain foods. The ones that did survive (birds, crocodiles, turtles, sharks, and small mammals, for instance) are notorious nowadays for being general specialists, being able to eat practically anything put before them.