Verilog-mode is a bit unique. It's not just a programming
mode that is used inside Emacs, but a computer aided design
tool that extends the Verilog language, relied on in most
every chip design company, and often run as a standalone
tool from Emacs batch.
Thus there are often important bugs which need to be fixed
back into what would otherwise be considered "old" Emacs
versions. cc-mode and like doesn't have this issue.
Also because of this, verilog-mode.el is often part of the
source code control system of design environments, which
need to run regardless that different users and systems may
have different Emacs versions installed.
Users often upgrade verilog-mode.el themselves, and so
having a single file, that they can easily install and know
will work, no matter their version of Emacs, is critical.
Perhaps this path is more development work (though due to
version maintenance effort I'd argue that). But, I don't see
any way in which splitting versions nor splitting files
would help the users.
Anyhow, regardless of these choices, new Emacs features are
made available. By no means is the least common denominator
all that is supported.
-Wilson