I guess it is because after decades of use, grep has probably been fixed to handle lots of user cases that the new tools don´t handle because they haven´t found them yet.
Like automatic encoding detection and transparently searching UTF-16?
Or simple ways for composing character classes, e.g., `[\pL&&\p{Greek}]` for all codepoints in the Greek script that are letters. Another favorite of mine is `\P{ascii}`, which will search for any codepoint that isn't in the ASCII subset.
Or more sophisticated filtering features that let you automatically respect things like gitignore rules.
Those are all things that ripgrep does that grep does not. So I do not favor this explanation personally.
ripgrep has just about all of the functionality that GNU grep does. I would say the two biggest missing pieces at this point are:
* POSIX locale support. (But this might be a feature[1].)
* Support for "basic" regexes or some equivalent that flips the escaping rules around. i.e., You need to write `\+` to match 1 or more things, where as `+` will just match `+ literally.
Otherwise, ripgrep has unfortunately grown just about as many flags as GNU grep.
