1. As I explained, the schedule of future ABI breaks was unknown when KDE SC 4 was released. There was no guarantee the 0.10 series would still be readily available over the lifetime of KDE SC 4, nor could KDE developers force Gstreamer developers to keep binary compatibility if they decided to break it multiple times over the course of KDE SC 4.
2. When KDE 4 and phonon were being planned, Gstreamer 0.10 wasn't even out yet.
3. Qt is a core part of a huge number of applications. Continued packaging of it by distros is guaranteed. A multimedia framework like Gstreamer, on the other hand, is much less important so is much less likely to be maintained for a long time. Add to that the fact that Gstreamer wasn't all that popular or well-supported at the time of the KDE 4.0 release (not to mention several years before when it was being planned), assuming that distros would want to continue packaging an unknown number (at the time) of incompatible versions of gstreamer for an unknown (at the time) amount of time. I hopefully don't need to remind you that unlike Qt 3, Gstreamer 0.8 is no longer being packaged by most (if any) distros. If Gstreamer devs had decided to do a 0.12 early in the KDE SC 4 life cycle then 0.10 would probably not be packaged, either.
4. Even if Gstreamer 0.10 continued to be packaged, the libraries it relies on are changing rapidly and breaking binary compatibility much more often than Gstreamer itself is. So without continued maintenance, something like Gstreamer rapidly becomes useless. There are already incompatible releases of Gstreamer dependencies that will never work with Gstreamer 0.10.
5. Gstreamer devs agreed with this approach and promised to write their own Phonon backend. However, around the time KDE 4.0 was released, they changed their minds, said they were completely opposed to Phonon, and never wrote the backend they promised. By then it was way, way, way too late for KDE devs to change their plans.
So yes, by shear coincidence it turned out that the only binary incompatible release of gstreamer happened close to the release of the next binary incompatible release of KDE, and Gstreamer is popular enough that the 0.10 series will likely still be packaged by that point. But there was no reason to think that was the case 5 years ago when KDE 4.0 was released, not to mention 8 or 9 years ago when it was being planned.