Lunars don't want to give it to them? and are the only ones who know about them, that and I'm guessing no one really wants a bunch of silver tattoos on them unless they like the aesthetic or really need them, but Solars already have a defense against shaping effects while Sidereals……the Lunars aren't on good terms with Sidereals.

that and, who would want a bunch of tattoos giving them away for the Wyld hunt to go "oh hey a Lunar anathema, attack." without the Lunar's special "hide the Silver tattoos" charm.