Why no USB host on PDAs?

Since more than 10 years all PDA makers implement their own connector to connect to PCs and peripherals. And to make matters worse, even if you buy your PDAs always from the same manufacturer, as they change connectors on a regular basis, there is a good chance, that the peripherals and cables you bought for your last PDA won't fit on the new model. Bluetooth is great to connect devices without wires, but for charging purposes you still need wires. So why isn't everybody using USB? What a shame. Life could be so much easier...