To be honest, I'd attribute it to flash versus fundamentals. A common trap that players fall into is the belief that the ability to preform advanced techniques makes you a better player. While this is true to an extent, in that having more options at your disposal can give you an advantage, what many tend to forget is that learning how to do something doesn't mean much unless you take the time to learn why and when to use something. B reversals are a great example of this, and with characters like ZSS it is an incredibly useful tool that can overall make her less predictable. When someone first learns to use it however, you are basically going to see it used every time they throw out a neutral b; commonly out of a pivot, dash jump away, and b reversal towards their target. While this is the right thing to do in some cases, there are many times where you could achieve the same result with a simple standing neutral b. Even then, being that they do it every time, it becomes predictable and actually puts them at a disadvantage. And even then, they won't make use of it as anything other than a tool for attacking with their neutral b. So, even if they've mastered the execution of a skill that you haven't, you can pretty reliably guarantee that they won't use it as a means of mixing up their descent to the stage, as a method of setting up for a ledge trumps, or with any move other than their neutral b for that matter; all they are doing is autopiloting. If they can't use it flexibly, they are playing no better/ differently than you are.

Now someone who focuses more on follow ups and learning bread and butter combos on the other hand is someone who is focusing on fundamentals over flash. Again, with ZSS as the example, if all I do is her neutral b, grab, down throw, up air, up b string, I've got a solid combo that builds a lot of damage and even kills at higher percents. Even if that's literally all I go for the entire game, that means I'm learning not to recklessly throwing out attacks that can be punished, I'm just focusing on what you're doing, waiting for you to do something that I can punish. At the same time, it's also forcing me to pay attention to how you behave in relation to what I do, so if for instance I notice that neutral b to dash grab isn't working because you shield the paralyzer and spot dodge the dash grab, eventually I'll learn to either set up the grab with something other than paralyzer, or to change up my grab timing so that I do something like dash past you as you spot dodge and pivot grab all that I catch you as it ends.

The point is, learning her bred and butter first means that I have a strong offensive option around which to base my growth, rather than a movement option that is only really useful once you have developed strong fundamentals and can use it flexibly. Even if we both autopilot, my bread and butter gives me a greater advantage than something like b-reversals, simply because bnb's kill, even if just trying to hit you; at best a b-reversal will save me from death when used in the right situation, or set me up to kill, but only if I know how to follow it up.