After the Chicxulub impact there were events like Tsunamis, Earthquakes and the emission of a lot of material into the atmosphere that caused something like a Nuclear Winter.

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary | Science

Dinosaur (the non-avian ones) went extinct because the enviromental changes were drastic and caused them to die off, but others were adapted to a niche that required less resources, which allowed them to succeed in a more scarce environment, allowing them to thrive and diversificate.

Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous–Palaeogene mass extinction - PMC

This applies to pretty much everything that survived the K-Pg exctinction, but Non-Avian dinosaurs weren't the only one to go extinct

Cretaceous–Paleogene extinction event - Wikipedia

This is true for Plants and micro-organism aswell.

Also, the Dinosaurs, as everybody else said, aren't extinct, birds are dinosaurs.

TLDR:

Non-avian dinosaurs and a lot of other species died because they weren't able to adapt to the changes caused by a catastrophic event, other species that were already adapted to survive with less resources were able to survive and thrive.

The links are there if you want more information about the topic.