Sure, but some (Ubuntu, for example) are just undesirable as a distro to run on an old hardware. Some don't even support old hardware, say the 586 archtecture

Sure, but some (Ubuntu, for example) are just undesirable as a distro to run on an old hardware. Some don't even support old hardware, say the 586 archtecture (Pentium). More important so, distros have different goals and mentalities. I don't think it would make sense, for example, to strip Ubuntu, for example, to make it run on a very old hardware. So, it's not _any_ distribution that you _can_ or _should_ run on older hardware. Some just don't suit.