First, I really like Eric's answer for practical reasons. But there is another side to screw head diversity: Security.
In the end, it's usually a temporary measure. Once a company invents a new screw, some third party will start selling tools to remove them. But for a time, any new screw design means the only people who can open your parts are "certified" workshops you've given tools to. Personally I see this a lot in electronic equipment, where the original manufacturer wants some aspect of design kept private or doesn't want after-market mods made to things they sell. Nintendo is particularly active in inventing new screws.
Examples of security screws include 3-prong Torx variants, and Allen heads with a raised dot in the middle of the screw head to stop standard Allen wrenches from fitting.
If you'd like to see a much more industrial version, find a car which has been booted and look at the bolt. I've seen circular bolt heads with an off-center circular hollow inside. No shoulders or slots to grab, you have to have a very specific wrench. And, of course, there are security lug nuts for car tires.