On behalf of Bart -
I find it a very strange question to ask.. No one would ask why IPv6 and IPv6 are both considered a network layer. This is why they were made for.
I believe the question should be what is the difference between SCCP and MTP3. So the SCCP was developed with MTP3, obviously after the MTP was developed. MTP was designed to carry circuit call related signaling, where with the SCCP it is capable to handle the noncircuit as well.
The main difference I would say is that SCCP is doing end to end routing where MTP3 is doing point to point routing. While MTP L3 can only identify and distribute messages to specific points and user parts, SCCP can route messages to specific applications that reside at signaling points.
On top of that SCCP is OSI Layer 3 compliant, in theory it can be transmitted over any OSI-compliant network.