dear kumar,
i love that pizza scene its outstanding example,let me try if u are not cleared any more.
1- SCCP does not have transportation mechanism so for downstream MTP facilitate it and for upstream TCAP do the same job.
2- Why TCAP? bcz at sccp level u knows that there is a part SCRC(SCCP Routing control) whose function is only to identify that which which application to be triggered for this call(INAP,MAp etc…) but it only points to the application to be triggered,now it is the duty of TCAP to deliver that specific call to that Application.
so simple.
i think its enough for your understanding..