You have the blame misattributed: almost nobody used client certificates because they cost money ($100+/year). That meant there was little demand outside of a few spaces like government and absent usage there was not much pressure on the UI improvements.
Client certificates are also worse for privacy and phishing resistance: with a certificate, if I can convince you to click on a link I get your identity. From the site's perspective, I don't have any way to tell whether the person with the certificate is the same person I saw or the person who compromised their computer or convinced a CA to issue a cert for someone else. Requiring key storage to be on a hardware enclave significantly reduces that risk, allows for the stronger attestation requirements mentioned, and also means that you're changing things from “trust anyone who can get a CA certificate” to “trust anyone who can do signatures from a previously-registered hardware key”.
Client certificates are also worse for privacy and phishing resistance: with a certificate, if I can convince you to click on a link I get your identity. From the site's perspective, I don't have any way to tell whether the person with the certificate is the same person I saw or the person who compromised their computer or convinced a CA to issue a cert for someone else. Requiring key storage to be on a hardware enclave significantly reduces that risk, allows for the stronger attestation requirements mentioned, and also means that you're changing things from “trust anyone who can get a CA certificate” to “trust anyone who can do signatures from a previously-registered hardware key”.