first of all, I will show you a simple example, suppose you have two computer in the network, if you observe in the IP layer, you can find all the packaet is the same for they have same source and dest ip address, yet when you see it in the layer above, you will find two different packet the TCp and udp. maybe sccp is somewhat like ip, and the tcap is somewhat like the tcp.
now let’s look at the sccp and tcap, suppose you can observe the ss7 message from a vlr to a hlr, if you observe them in the sccp level, you will find out that all the packet is the same, for the source and dest Gt is the same,you can not distinguish each packet. but when you observe them in the tcap level, you will see they are all different by the variable tcap id, what is more, if you catch a packet with a certain tcap id, you can also find some other tcap packets which have same tcap id. all these packets are delevered in sequence to work for an application for a certain subscriber.
now we can see sccp works for the message transfer between network nodes, and tcap works for a special application defined by its rule.
in one word, they are desined for different purpose.