Fragmentation makes compatibility and help-finding more difficult. And newbies do get the expectation that the mainstream options aren't all that different, cause that's what everyone tells them.
People do have difficulty switching between Mac and Windows, but each has critical mass so it's still easy to get help with the finer details. And unrelated to fragmentation, anyone tech literate won't have nearly as many showstopping issues to ask about there in the first place.