They can only go to unaware characters, who need a reason to get them... and some idea of how they're going to cope with having a bigass shield or two-handed sword around, because they're still complete (it's why they were so easily found).

Excalibur's requirements come from it being the wielder's second relic after the first was gone--so rather than simply affecting how it manifests, their experience (and additional power) imprinted something of themselves as a requirement. So... pretty much just an expectation of a chivalrous nature. Also being British.