Because at SBB, a train coming from DLI and bound to CNB has to cross all the way to the right. An MB/MTC bound train coming from NDLS has to cross all the way to left. This crossing can be done one at a time unless we have a NDLS-CNB train and DLI-MB/MTC train which may go parallel unhindered. In all other combinations, one will have to wait for another.
This further gets complicated if there's a train coming in opposite direction. Imagine a train coming from MTC route which has to go to NDLS. Untill this train crosses the SBB junction point, a train coming from DLI or ANVT can't proceed ahead. So it's all clogged.
And...
more... now you know why the Rajdhanis zip pass in relatively lesser time. They come from NDLS (right side) and proceed to CNB (right side). No cris-crossing.