When i started slackel on 2001, i had used kde. It was light then. When kde become hungry on resources, i developed slackel openbox. Window Managers is a matter of taste. I prefer openbox than fluxbox. I like mate also. Others prefer kde. That's why, even if i don't use kde, anymore, i provide a kde iso for users want it.
To maintain a distro, which has to be stable also, using many window managers is tired and need a lot of time. I use slackel openbox which is light for my personal everyday use. Even on slackel which is based on slackware current, the time used for solving problems is too much. Things can break easily after an upgrade on slackware current.
Of course it is a good idea to provide lxde, mate and xfce slint isos. If you have the time to do it.