Why did many plants die at the end of the mesozoic era?

Dust from the meteorite collision obstructed sunlight is the right answer.

Based on the general argument that the dinosaurs went obsolete due to the result of a massive asteroid, experts consider that the asteroid produced large amounts of trash to float into the environment, obstructing the path of the sun. This resulted in global temperature decline which caused the plants to freeze. Moreover, the process of photosynthesis could not be carried out in the absence of the sun. Therefore, the absence of the sun at the end of the Mesozoic age caused the death of plants.

not only is it colder, it also becomes darker, less salty, and more dense.