My take on it, right or wrong, is that both instruments taking all parts makes the song too long, and possibly repetitive. Also, B parts can be less melodically interesting, and if you're not playing an instrument that is also percussive, the B part can just be boring.

The B part to J'etes Au Bal bores me to tears when I play it on fiddle.

I'm sure there are exceptions to what I'm saying and what Jude is saying.

I think there's a more simple evidence proved explanation :

Acordion players are sober over-trained super-healthy athletes who can play all night long without suffering tendinitis or muscle pain, so they play both A & B parts even the D or the Y-Z part.

At the opposite, fiddle players are brittle drinkers, so if one turn they go to the bar , next turn they go to the toilets. CQFD.

Sorry for this one
Christophe

I don't know you, but I don't trust you!

I think Cajun music has become way too regimented. My opinion only, but the accordion player should play what he wants, then pass to someone, and that person should play what they want and how long. Marc Savoy and Michael Doucet do this.