Simple. If any Shatabadi Takes are replaced by VB trainsets, a Large no of LWSCZAC coaches will be released.
Now Normal Overnight Trains have a large no of AC coaches, they can absorb coaches freed from Rajdhani trains.
But,...
more... Daytime Express trains usually have a very limited no of AC coaches, and they will take a long time absorbing the AC coaches freed from Shatabadi trains.
Also LWFCZAC are used in a VERY LIMITED no of non Shatabadi/Tejas trains like Jaipur DD, steel Express, etc.