The iPhone 3G had a polycarbonate ("plastic") back. One of the major complaints was that this shell would inevitably crack over time. For this reason, when Apple went to the 4, the phone was a sandwich of Corning glass with a metal frame. The problem here was that the glass, while strong, was thick, heavy and brittle. So when the 5 came out, the new shell was aluminum. Now, with the 5C, we are back to a polycarbonate shell, except its supposed to be an inexpensive consumer throwaway device. If you want something to keep for a couple years, you can go buy the 5S.
All the other phone manufacturers are selling devices that are complete junk within a year, either through (poor) design or obsolescence, so they don't invest as heavily in the materials as Apple. They also don't seem to be investing in support either, since quite a number of phones rarely (if ever) see an Android update beyond the version it's sold with. And most consumers seem to be quite content with that, opting instead to buy a new phone every year.