。。
She is very kind. That's why everyone likes her.
Literal
She [topic-は] very kind. That [subject-が] she [subject-が] everyone [by-に] is-liked reason is.
それが彼女が...理由だ is a 'reason cleft' — the structure highlights that the previously mentioned trait is precisely the cause of what follows. 彼女が好かれる is a relative clause modifying 理由, with subject-marker が (rather than は) inside the embedded clause as relative clauses normally require. The double-が pattern at the front (それが + 彼女が) is unusual and marks both the deictic 'that' and the embedded subject.