A lot of people don't really understand computers. We mostly know how to manipulate the user interface (UI) to get the computer to do what we want, but if you switch up the icons or install a new desktop environment, I guarantee you that 99% of users will be completely lost.
This is because the UI abstracts the complex process of running a computer so that the user can just think about getting things done. The user doesn't need to know what it means to "go to the start menu and click the Notepad app." Practically, this is Windows-speak for "open the default text editor". However, if you take a Windows user and drop them into a Linux and ask them to open up the default text editor, they probably won't know how to do that unless a Windows-like desktop environment is chosen.
Basically, a lot of people don't "know how to use Reddit" so much as that they know how to get the Reddit website to do what they want it to. Lemmy is even slightly different than Reddit, cosmetically different. Although we usually use the phrase "cosmetically different" to imply that the difference is not important, because we rely on GUIs to understand computers, cosmetic differences are really important in UIs.
Go look for posts on Lemmy discussing Jerboa and the other apps. The apps mostly differ in how the user is able to interact with the site. They should all have the full functionality of Lemmy (or are working towards it), but the ways of presenting that functionality to the user are different.
One of the most important groups that moved to the Threadiverse were the blind community. It is because of the inaccessible user interface in the Reddit app that they decided to move over.
And let me be very clear that the fact that computers abstract away their complexity is very much a good thing. That's why we have computers: to do tedious, complex work automatically and simply. Not everyone needs to be a computer expert, but I do think that developers need to resist the urge to make cosmetic changes that don't improve functionality. I realize that this is an ill-defined tall order. Regardless, we need to be aware that most people don't know how computers work.
I think that, in order to get people joining our communities, we should try to be compassionate and helpful when it comes to users learning how to use site. Actually, this is a special case of my more general position that we should try to be compassionate and helpful in the face of people who are confused and trying to learn, whatever the subject. I know it can be hard; if I'm being honest, I have a bad habit of getting annoyed at people who don't look like their listening. But we need to unlearn that.