What are the reasons that you can think of for the arthropods to constitute the largest group of the animal kingdom?

The phylum, Arthropoda, consists of more than two-thirds of the animal species on earth. The reasons for the success of arthropods are as follows.
 Jointed legs that allow more mobility on land
 Hard exoskeleton made of chitin that protects the body
 The hard exoskeleton also reduces water loss from the body of arthropods making them more adapted to terrestrial conditions.

Arthropoda is the first phylum in which animals have properly developed organ systems. Elaborate organ system helped the arthropods in surviving in diverse conditions. Moreover, the arthropods are the earliest among the animals with well developed organ system. These are the reasons that arthropoda is the largest phylum in the animal kingdom.

Arthropods are the first phylum to have well developed systems to carry out different activities. There is distinct system for respiration, locomotion and reproduction. Their survival capacity is great because of elaborate system. This has helped them survive in diverse conditions. They can live in water, on land and in air. 

This can be one of the reasons why arthropods are the largest group among the animal kingdom. Another reason is their early development compared to animals of higher phylum.