Here's why:
* Oldest: Insects are the oldest group of land animals, appearing in the fossil record about 480 million years ago. This predates even the dinosaurs!
* Most numerous: Insects make up over 90% of all known animal species on Earth. There are estimated to be over 9 million species of insects, with only a fraction of them having been discovered and described.
Other animals that have been around for a long time include sponges (Porifera), which are very simple animals, and cnidarians (jellyfish, corals, anemones), but neither of them come close to the sheer diversity and longevity of insects.