Because up to that point animal life was closer to sponges than anything else. Maybe a moving blob here and there in the ediacaran. Then shit evolved predation, mobility, pursuit sensation… And so what does the blob do? The simplest thing that by chance can change in its dna. And what is that? Hardened spikes with no real anchoring or repurposing. Just swords sticking out of a brain.

Meanwhile someone else is evolving to float around and have the majority of its body mass by stingers and someone else is becoming all of bugs

Great question! Do some science.

I'll give you a starting place. Consider their mobility, camoflauge, protection, abilities of their predators, habits of their prey, and mating strategies.