The advantage of EPS over PDF? And why so many journals ask for EPS alone?

The eps format used to be the only possibility for including graphics in (La)TeX. Now the more modern alternative is using pdflatex, which also supports other more common graphic formats natively (such as pdf).

Most journals have outdated LaTeX processing pipelines, so they are still using eps. (And, frankly, they have little incentive to get more modern, since they are in a strong oligopoly position.)

The most probable reason is that many publishers use Adobe products (such as InDesign) in their production and these used to rely heavily on EPS. This might change in the future as PDF becomes more popular.

Also, EPS is an open format, which means any graphing or vector graphics program (like Inkscape) have options to save artwork and plots in EPS. Not to mention Matlab and the like.

Eps is a format designed specifically for printing. All publishing companies have eps and ps editing capacity .