my assumption throughout the first several years of iOS was that the aim was for devices to have two major updates and that would be it.I guess the engineers didn't want the 3G to run 4.x either, but marketing had decided that it was too early to cut off support. At a technical level, surely the footprint is roughly the same for both devices.
And that’s pretty much how it was, until 2012 when the 3GS broke the mold and received another year of updates.
But certainly they did have a pattern going for a while, the original iPhone and iPod touch got two years, as did the 3G and Touch2, and the third generation iPod touch, and the original iPad, and even the fourth generation iPod touch.
At the time though it was pretty much the best that was around, especially since the earliest Android phones were usually limited to the operating system they shipped with.