When you go on CRAN page for Rcmdr you'll see that it depends on R > 3.5.0.
This is why, the package is not available to you - you should not find it in the table available.packages() that filters by default according to the R version. See ?available.packages about built in filter.
As you are on R version 3.4.4, you'll need to install a former version. Previous one, 2.5.0 should be fine according to the DESCRIPTION file from this version
Unfortunately this isn't working for me, I can install R Commander version 2.5.0 using that command, but it then attempts to install package 'rgl' as a dependency. This installation fails, with the message:
SystemRequirements: OpenGL, GLU Library, XQuartz (on OSX), zlib (optional), libpng (>=1.2.9, optional), FreeType (optional), pandoc (>=1.14, needed for vignettes)
You need to make those available on your system.
About dependencies, normally, remotes::install_version("Rcmdr", "2.5.0"), as I mentionned in my solution, default to not installing suggestes . Maybe you ran remotes::install_version("Rcmdr", "2.5.0", dependencies = NA, dependencies = TRUE) that will also install Suggests (or there is a change in default value)
As rgl is in suggest, you may be able to not install Rcmdr without it using the correct argument for dependencies