Translations:Initramfs/Guide/20/en
From Gentoo Wiki
The content of the initramfs is made by creating a cpio archive. cpio is an old (but proven) file archiver solution (and its resulting archive files are called cpio files). cpio is definitely comparable to the tar archiver. The choice of cpio here was because it was easier to implement (code-wise) and supported device files which tar did not support at the time.