VLSI Fabrication: Why Aluminum and Copper used for metal layers?
http://en.m.wikipedia.org/wiki/Copper_interconnect provides some answers:


aluminium was easier to deposit; a new process had to be developed for copper.
copper ruins silicon's semiconductor behaviour if it leaks into it, so part of the process development is sealing to keep out the copper.
the conductivity benefit isn't as big as you might think.


However, in modern high power consumption devices, it's become important to have that extra few percent conductivity. So effort has been invested in making copper work.