This has to do with lots of factors. But definitely there is more than just the formal aspect of a typeface (the design). I once attended a lecture where a guy tried to explain how an art piece can become a classic. He took Mona Lisa as an example. Of course there are some formal aspects in the painting (style, composition) which were revolutionary at the time it was painted. But this is not the only (main) reason why it became such a legend. Not so long ago it was even not part of the top 10 list which the Louvre defined to be the most important paintings of their collection. Only with the rising popularity of da Vinci as a person and the fact that the painting was stolen and lost for a long time, the painting got so famous as it is now.
Another nice example he mentioned was also The Girl With A Pearl Earring from Johannes Vermeer. The painting from 1665, was sold only 125 years ago for 2,30 guilders (1 euro you can say) because nobody really saw much value in it. But now it is treated to be a very important painting. How can something like this happen? No idea, but it's not only the painting itself.
He ended his talk with the diamond-skull of Damien Hirst (For The Love of God). He was discovering some aspects which could make this work to become a classic: uniqueness & little scandal. Damien Hirst is probably the only artist alive which is able to get such an amount of money to produce one single artwork. And this aspect is probably the only possibility to get a scandal these days. But of course there are lot of other works of arts which produced a scandal, but got forgotten in the meanwhile. So there has to be more. The guy suggested that to become a real all-time-classic, there should be a little secret included in the work. And he was sure that Damien Hirst also thought of this aspect. The guy ended his talk with the vision: There will be a day in the future where Damien Hirst is telling us whose skull it is, which we are looking at all the time. And from this day on this work has a very good chance to become the new Mona Lisa.
I think something similar goes for typefaces. Not identical, but similar. Helvetica is treated to be a classical typeface by some people already now. I think it needs more time. There is a good chance that in 100 years from now, Helvetica will be treated as oldskool (and nobody will use it anymore). The same can go for Meta or Scala or whatever typeface. The older the typeface is, the more the chance that it is a real classic. Now. And in 300 years. Maybe.
To become a classic typeface, I think some things have to come together.
They could be:
• Strong design (kind of new, surprising, high standard, good in details)
• Strong personality of the designer
• Strong story about the design of the typeface (purpose, context)
• Some magic