People seem to be forgetting that Tataru changes class multiple times during the MSQ, though in her case it's only crafting/gathering classes.

I think the implication is that everyone can change their class/job at will whenever they like, but the WoL is able to learn at a massively enhanced rate due to their affinity with the various soul crystals they possess. Chalk this up to the Echo; it gives us the ability to learn from the memories stored in soul crystals really easily.

For other characters learning a class/job is like a lifetime investment so very few ever branch out to more than one or two.

TL;DR The Warrior of Light is special!