There are as many theories why the dinosaurs are no longer with us as there have been minutes since they departed this plane.
Two main ones:
The Louis and Walter Alvarez study that indicated a possible extraterrestrial impact (asteroid or comet) causing what was effectively a nuclear winter, with plankton dieoff, carbonated oceans, frigid temperatures worldwide, etc. Supporting evidence indicates a possible 200-mile-diameter crater centered on Chicxulub, Yucatan, Mexico.
Widespread volcanic eruptions causing similar phenomena. Supported by the eruption of the Deccan Traps, a multi-thousand-square-mile lava flow in peninsular India, dated to about this time.
Other hypotheses include ontogenesis (they evolved contrasurvival “adaptations” such as unwieldy size, a la Irish elk antlers, and died from them), loss of interest in sex, poisoning from alkaloids in angiosperms, racial senescence, etc. Then there are those that don’t believe they were killed off by anything catastrophic but were on their way out, and one of the above simply finished the last survivors off.
I’m inclined to believe a combination of the two main ones is responsible, but with inadequate evidence to support this view. Anybody got additional information? Any other WAGs that deserve posting?