With the hardware in the present iPads, it's not possible to natively support USB ports. With Android tablets, the hardware is devoted for extreme multitasking and supports USB ports more easily because it can donate an entire core or two cores just to the data transfers, plus the battery and RAM allows for it. On the downside, if your USB is infected by first plugging it into an infected computer, then onto your device, you openly allowed for the virus/spyware/malware onto your device, and because sandboxing on Android is not as same as the heightened/multi-layer sandboxing that's present on iOS devices, it's a high-risk addition.
Apple's idea is to pack light and promote Cloud services. iCloud in itself isn't exactly the ideal example of "good" or "usable" Cloud service since it's limited in what you can host, but that would point towards services like Google Drive, DropBox, Box, Exchange Servers, VPNs, etc, which the iOS has semi-control over what goes in and out of the device. The reason would be because of security purposes, so that you're not openly introducing unwanted items to and from your device to protect your privacy and sensitive information. I agree that a USB port or some sort of add-on would do wonders on the iPad yes since 16GB flash drives runs you around $10 if not less compared to half a decade ago where it was $50/60 for medium/high-end high speed xfers like the Patriot Xtreme, but it does not allow for secure transfers of data to and from the device, and does not do well with travel as you can drop it. Possibly if they end up creating an Apple-exclusive type of chip that allows for innate sandboxing of data that requires a TouchID scan plus several layers of passcodes to eject out of the device, it may expand on the storage/sharing capabilities of iOS