there are two main reasons:
during the 1980s when Airbus Industrie developed the A320 Toulouse (TLS
) was the company's main plant where every plane (they only produced A300s and A310s then) was assembled. Therefore the A320 was asselmbled there, too.
the plant at Hamburg-Finkenwerder (XFW
) doesn't have the capacity to produce all the A320s beside the A319s, A318s and A321s, so until now no effort has been made to transfer the final assembly of the A320 to XFW
And of course Airbus Industrie (with its headquarters in France) doesn't want to produce its most successful plane outside France ...