The question then is why DESeq2/limma/edgeR etc were R packages, rather than python packages.
The answer is that they were created by with a statistical background working in biology, for the use of people with a biology background rather than being created by people with a computer science/software background.
Stats people like R because it was written by stats people to do stats in. It has first class support for table data and statistical models. It has fast, built-in code for fitting statistical models. It thinks the way a statistician thinks.
It also has a long history of use by people who class themselves as being "non-coders". Long before there was literate coding and notebooks, typing at the REPL and evaluating the output (including plots) before deciding what to do next was a common way of working in R. This appeals to people wanting to think of it as a piece of software for analysing their data, rather than a language they have to "program" in. Finally R has a long history of all packages having very detailed step-by-step tutorials in the form of "vignettes" that have first class language support.
I just would like to understand why R analysis prevails in RNA-Seq analysis, since I'm studying Python and would like to do all of my projects with the same language.
I'm afraid you can ask the same question for any language.
90% of the posts in Biostars about RNA-Seq analysis talk about EdgeR, GSEA, and other R packages.. is it worth to learn Biopython, if I'm more familiar to Python than R?
Python in bioinfo analysis is "fairly" new, while R has been around since forever. That's why the majority of core rna-Seq analyses are done in R. If you want to use python, a lot of packages have already been ported (e.g. PyDESeq2), or alternatives packages are available. If you prefer python and the libraries/methods you need are available in python, then don't worry about R.
there is pyDEseq2 if you want to stay in python https://pydeseq2.readthedocs.io/en/latest/ people commonly use DESeq2 in R