The main reason for extinction of species is

Habitat destruction is the process in which the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction is the greatest contributor to the extinction of many species. This destruction is ongoing in both terrestrial and aquatic biomes, with approximately 80% of all extinctions being attributed to human caused habitat destruction. Terrestrially the destruction includes forests, deserts and grasslands, but the latter may account for the greatest losses due to the agricultural and urban conversion.