There are some cases where it's just attitude. Like I feel this person was just blasting her resume to everyone and picked someone whose cover letter felt a bit more sincere.
There are plenty of cases where someone is simply not qualified. Or that I'd get the impression that this person is not very smart and choose not to move forward with the interview process.
Weak English is another major reason. We rarely need someone good with algorithms or a specific tech skill. This is all learnable. What we really need is someone who can build what we want the first time, and not spend 2 weeks building the wrong thing. Or having to hire someone else to read the manual for them. It's really hard to explain why you'd think someone has poor English. Grammatical errors and a few typos are okay. But sometimes it feels like some effort to simply communicate with the person.
Also do note that the person on the other end is human. We don't really want to say why we reject someone, as the reason is often unfair and false. It's very uncomfortable to explain it. Easier to just ghost someone.