Why some papers are retracted and others not? Recently, in Reviews of Modern Physics, I have read an erratum which represents an apology for a copied verbatim text. The text is:

"Large parts of Sec. VI.A, “Inverse magneto-optical excitation of magnetization dynamics: Theory,” consist of text
and equations that are verbatim quotes, or very nearly so, of material in the paper by Gridnev (2008). Although this theory
was developed in close collaboration with Dr. Gridnev and jointly published in Kalashnikova et al., Phys. Rev. Lett. 99,
167205 (2007) and Kalashnikova et al., Phys. Rev. B 78, 104301 (2008), we adopted the more compact and rigorous description
given in Gridnev (2008) for this review. We regret that we did not identify these parts of our review as direct quotes, and we
apologize to V. N. Gridnev and the readers of Reviews of Modern Physics for this oversight."  from here. 

Although there is a copied text, from what I see, there is not a retraction. The question is: why this paper was not retracted?
In general, an editor will choose to issue a retraction when misbehavior or errors undermine the "core" of a paper (authors may request a retraction, but ultimately it is generally the editor who decides).  An erratum or correction, on the other hand, is for when there is a problem that doesn't change the "core" of the paper.

Different editors will draw the boundary differently, and some things are in a grey area.  In this case, it looks like reuse of background material (a grey area on self-plagiarism) shaded over into plagiarism from another closely related piece.

In most cases plagiarism, even of a background section, would be enough for a paper to be retracted due to the issue of dishonesty, which cases doubt on everything else in the paper.  In this case, however, it appears that given the close relationships between the authors and the papers, the editor judged it less a matter of theft than of sloppiness, and opted for the lesser correction.