Quaternion’s are the default because it’s easier for new people to use them without having to learn about Gimbal Lock. Simple enough.
It’s not easy to deal with gimbal lock when you begin, ok.
It’s a little hard to understand, ok.
But I’d argue that:
First, you don’t need to understand the phenomenon itself, so the actual “hard part” is completely facultative.
Secondly, you’re not supposed to deal with that as you begin: you just animate your bouncing ball on a 2D plane or you rig a sausage. When you move to more advanced stuff you already have mastered the ground competence & knowledge in order to get how to manage gimbal locks anyway. And if you have a hard time dealing with it as a beginner, then most likely the issue is from your learning path before anything else.
Lastly, using quaternions also have its own quirks that require both rigger and animator to know how to deal with them. So at the end of the day, you need to face a hassle no matter the rotation mode.
But I think that overcoming the gimbal lock issue is way worth it, compared to constantly having three values that unlike anything you’ve learned so far don’t seem to be linked to the objects actual transform rotations so you can’t even figure out what the heck happens in your coordinates and graph editor, and you can’t make revolutions nor straight predictable paths, and interpolations do not reflect as intended in the viewport.
Productions don’t have this problem at all (at least those that are not built by people stuck in a bubble since 2004). More often than not, they either have riggers who know which rotation order to choose, or they have tools to convert rotations orders at anytime while preserving the poses, or have rigs that are built to offer additional controls where gimbal locks might occur. Or all three at once. But one thing remains: they use Euler. Because that’s the one rotation mode that allows you to actually understand what’s going on in your graph, and that’s what animators need.
Though I might have an answer to why quaternion is the default: when you build a rig, you either choose quaternion or one of the six Euler rotation orders.
It is not a choice between “quaternion” or “Euler”, but between “seven different options” that each could potentially be used.
So let’s say the default is some Euler mode. If you do need Euler, you statistically are more likely to choose a different rotation than the default one. In other words: even if the default was Euler, you will make a change most of the time.
So that might be why it’s the default in Blender.
That being said, I’d still prefer Euler XYZ. Because that’s the one that makes rotations and interpolations crystal clear when you begin, that’s also the one used by default on objects, and that’s the default in many other softwares too.
And I also wish that Blender had more tools to manage default settings. We only can choose the startup states, but not the default behaviors, and that could be an enormous dealbreaker to work on.
Up to the point when they try to figure out the noodle salad that quaternions causes in the graph editor and they will turn to euler again. Every professional character animator i worked with (including myself) prefers euler. I’m allways animate rotations in gimbal mode anyway, so gimbal lock just don’t happen to me.
Whats even more worse ‘quaternion as default’ is that Blender doesn’t provide you with a reliable Euler Filter. However they did something to it in 2.92 but i haven’t tested it yet.
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