This isn’t a simple thing tbh.

It’s partially the difficulty in keeping hollandaise and bearnaise shelf stable and unbroken without sacrificing flavor.

It’s partially cultural, in that mayo is what got popular at the right time for an effort to make it shelf stable and maintain flavor.

And there’s the versatility, though that’s largely a matter of perception. Since mayo is a thicker sauce in the form that gained popularity, you can do a lot more with it than a proper sauce that’s going to be more runny.

I mean, if you’re asking this, you’ve made hollandaise at least, and probably bearnaise. So you how that it can difficult to keep together in the fridge. It tends to break in a way that home made mayo just doesn’t.

If you add enough extra emulsifiers to keep it together through shipping and storage, then you mute the taste. It’s like you said, you can’t buy good hollandaise. It’s the buying part that interferes in making it a sauce/condiment for the people at large.

Since getting either one to a store that’s tasty isn’t currently realistic, it’ll never get enough demand for it to improve. Anyone tasting the store bought stuff that’s out there already isn’t going to be a fan. If they’ve ever had it in a restaurant, the packaged stuff is unpleasant in comparison (if only bland and uninteresting on its own merits). And, if they can make their own, they probably aren’t interested in buying it because it isn’t exactly hard to pull off at home. My teenager can do a passable hollandaise, and they don’t even care about cooking.