You will have to ask them for specific reasons.  As a general trend,  different distributions have over a period of time come up with various configuration files and formats to do the same thing which leads to pointless differences and makes it a fragile development platform for any third party developer.  Adoption to a neutral location is easier than asking them to buy into /etc/sysconfig on the whole because of the legacy involved, for both technical (non extensible file formats, distro specific quirks) and political reasons (why should we adopt a Red Hat or SUSE decision etc).  For distributions using systemd, a change like this is easier to adopt now because they have already bought into the benefits of systemd and compatibility has been maintained during the transition period. 

Rahul