I suspect Canon was focused on making the new zoom as small as possible....
The other possibility is that by reducing the zoom range they've been able to improve the overall optical quality over the 24-105. It's also possible that at f4 it could out perform the 24-70 f2.8 too.
If rumours of a Canon high mega pixel DSLR are true, they could do with a better performing mid-range zoom in their line up.
It will be interesting to see some real world tests.