The package 'cmake' only ships a README.SUSE file and serves
as a meta-package.

cmake requires cmake-implementation, which inside OBS is provided by
  * cmake-mini (minimal cmake variant, no especially no libcurl/libarchive)
  * cmake-full (what used to be called cmake before)

This complex setup was done in order to be able to eliminate build cycles, as
more and more tools were moving to cmake as build system, but with curl in the
build chain, was making it increasingly difficult to break the cycle.

cmake-mini is not meant for installation on end-user systems (where it also would
not save a lot; as an end user, you have libcurl on your system anyway due to libzypp)
and is thus not part of the FTP Tree.