The argument that is genuinely clinical
Where the gum over an implant is thin, colour travels through it. A titanium fixture and a titanium abutment underneath thin tissue can produce a grey cast at the margin that no crown material fixes, because the shadow is not in the crown. Zirconia is white through its whole thickness, so there is nothing underneath to show. On a front tooth in a thin biotype, that is a real and visible difference rather than a preference.
The other reason people ask is a preference for not carrying metal. That is a legitimate thing to want, and this practice does not argue patients out of it. It is worth being clear that it is a preference being served rather than a clinical deficiency being corrected, because the two get blurred in marketing and the blurring is what leads to a material being used in a case it does not suit.