I wonder why Debian didn't go with OpenRC (https://en.wikipedia.org/wiki/OpenRC) as it is compatible with both Linux and *BSD, which also offers 'Process segregation through cgroups'.
For OpenRC, there is just one person advocating it. There is a summer of code thing, but seems not much work is done. Furthermore, OpenRC seems nice, but parallel starting of services is actually experimental/buggy (just ask one of the OpenRC maintainers!). So although you can have a similar boottime, one init system (systemd) will be running in a supported configuration, the other (OpenRC) in something that is known to have issues.
That all said, I don't think Debian really makes decisions. What is being proposed is maintaining two.
Aside from above, there is also the practical bit in that systemd provides something to replace the unmaintained and deprecated ConsoleKit with. This bit can be used on another init system (Canonical is using or will use it on Upstart), but at one point seems easier to rely on the existing systemd supporters in Debian to get things working correctly.