Other phones do have a WiFi on/off switch on the outside, to make it easier to change.
Any time something is a common action, dedicated buttons are a good thing. Power and volume controls are in that category. Being able to instantly come up in dial / quick-dial mode is another common action, but was dissed because of the emphasis on touch only.
It also often depends on what kind of lifestyle the designers had. If they didn't care about battery life and/or always wanted to use WiFi when possible and/or didn't have problems with lots of undesirable hotspots all with the same SSID... then they wouldn't see it as a high priority.
These things take time for a company to figure out.