No one seem to want to touch anything that already exists. Never heard of anyone thinking of redesigning UI/UX for a typical MUA or browser's keystore (throwing in BTBV option or whatever), although I still believe that must be possible. Everyone's off with their own proprietary non-interoperable (occasionally, "open") standards.
Also, _almost_ no client supports _any_ form of authentication and encryption on mobile, be it OpenPGP, S/MIME, PEP, SaltPack or whatever else. There are few, but that's not even remotely close to "most". Neither there is much choice of good desktop client software as well.
iPhone supports S/MIME out of the box. Android has 3rd party mail clients that do. Several desktop clients do, including Outlook.
Its not as dire as you make it to be. People just dont want the hassle and don't value their email privacy. Once they do value this, which they should considering all the breaches of late, then it'll catch on. I work with companies that have S/MIME internally for just this reason. Its completely feasible.
Also, _almost_ no client supports _any_ form of authentication and encryption on mobile, be it OpenPGP, S/MIME, PEP, SaltPack or whatever else. There are few, but that's not even remotely close to "most". Neither there is much choice of good desktop client software as well.