My impression is "why" is political, with regulations like EU's eIDAS and other national digital sovereignty efforts setting the rules. It's not even WebTrust that's the target in most cases, it's the ETSI EN 319 series and ISO 21188.
Is it possible to convince the EU to say that, no, each one of your 27 member states isn't allowed to have a PKI? I don't think so.
Is it possible to convince the EU to say that, no, each one of your 27 member states isn't allowed to have a PKI? I don't think so.