User experience.
It is frustratingly bad for people just wanting to sign up. The idea of creating a single account, tied to a single instance that may or may not be federated is a confusing concept when most places online now are centralized. Then you have to choose which instance you sign up to, then depending on the instance you may need to go through an application or some other hoops to finally have a working account.
Then you may later find out your instance is not the community you thought it was, may be un-federated from other communities you had wanted to engage with, or the instance just shuts down one day, you loose your account and have to start all over again. That frustration and confusion is enough to turn away most casual and less techy users.
Also, there are lots of apps to pick from, but never a de facto one. While not necessarily a bad thing (in fact it can be generally good!) it does not help with the issues laid out above.
True now, but doesn’t have to be. Could have a site that aggregates and simplifies these steps to one place, allows you to pick and choose your instance with explanations right there to help you make informed decision on your account, and let’s you know upfront on which instances have other restrictions. Filters for those lists, and other quality of life features that people have come to expect in a modern user experience.
I know some have tried sites like this before, and I’m sure there are some kind people out there that may even have the handy excel sheet to share as well, but few to no solutions out there do it all, and certainly not all of it well.
All this is to say, there’s things the fediverse could have better, but it’s really reliant on volunteers to make it what it is and those are in short supply.