I already made two different questions about similar things, How could an efficient respiratory system evolve for giant arthropods? and How could an efficient circulatory and cardiovascular system evolve for giant arthropods?.
Those questions asked about the "how", but this is about the "why".
This is because I would like to have giant arthropods which require those adaptations in a potentially Earth similar enviroment, in possible convivence with vertebrates and other creatures inspired by questions in this site. This all without use of magic or genetic engineering, just natural adaptation and evolution.
So, what kind of enviroment pressures would drive arthropods to develop these adaptations?
Considering that a suitable environment is available for this, but what are the characteristics of this environment?