[JKT]

I assume it is because you could theoretically have ([9 x 16] + 4) = 148 apps all sending to the queue in quick succession (and potentially multiple times for each app, e.g. an IM app), thus leading to notification overload and battery drain. In theory that worst case scenario sounds absolutely dire; in practice, I doubt it would be anywhere close to that extreme. However, I suspect that once Apple started testing their initial implementation, they quickly realised that there was the chance that things could get close enough to the extreme that it would be unworkable in real world usage.

I'm just hypothesising so I could be well off the mark. However, what is self-apparent is that what Apple tried just didn't work well enough or we would have had it in the OS by now.