Direct Input isn't "lesser known", it's just not what has become the standard in PC games over the past generation. Microsoft's proprietary standard for 360 controllers is, unfortunately. Not that it's bad per se - it works great - but it means that controllers which use the "actual" standard, Direct Input, typically don't work in games without using wrapper software.
DirectInput is every bit as proprietary as XInput is. Clue's in the name, it's part of DirectX (semi-deprecated since XInput was introduced in DX9, but still present, and commonly used for mouse/keyboard/etc input).
What you might be thinking of is that most legacy controllers (and PS3/4 controllers, etc) will show up when connected to a PC as standard USB/BT
HID devices, which are handled in Windows by the standard DirectInput driver.
The trouble is, USB HID is great in terms of flexibility (as it was designed to be a generic way of supporting any input device, from keyboard/mice, up to full-blown flight sim HOTAS setups, etc), but that very same flexibility makes it a nightmare to work with in games, as there's no standardisation of button names/locations, stick/trigger axes, etc. So you can't ship with a standard controller input profile, as it would be nonsensical on most hardware, and players have to map buttons themselves, and you can't have any kind of useful button prompt, as what looks like "button 1" to the game might be called any number of things on the pad (might be an A button in a typical ABXY diamond, could be an X on a playstation controller, might even be the start button, etc).
