I think one important fact about the longevity of standards is that many if not most of them "came of age" during the early war years, which represented a "loss of innocence" in many ways for our country. These tunes became associated with feelings of national unity, perseverance, nostalgia etc.
They came out of popular movies. They became popular as a distraction from the everyday routine. The popularity of dancing certainly had a lot to do with attaching a nostaglic feeling to these tunes.
I think that tunes have to "ripen on the vine" quite a while before they enter the mainstream as "standards". I do think that there are tunes coming out of current pop music that may become standards. Not many, though.
I've pondered this question for a long time- what makes these tunes so special and timeless? Yes, part of it is the quality- composers like Jerome Kern, Cole Porter, Irving Berlin etc. seem to be quite rare these days. I mean, as tired as you might be of hearing "All The Things You Are" it's hard to deny that it's a beautifully constructed tune. Or "Stella". Or "Body And Soul". Etc.
IMHO, it's also the aging process that makes these tunes so immortal. Like a fine vino.