Renkonti is a transitive verb. For speaking about the act or the event of meeting, it would be shorter to use Renkonto instead of Renkontiĝo. Why is the word second more common?
In English, you can write We met in London as a complete sentence. In Esperanto, however, the verb renkonti is transitive, so Ni renkontis ("We met...") feels incomplete on its own: it is more complete as Ni renkontis nin or Ni renkontis unu la alian.
Otherwise, we can make the verb intransitive: renkontiĝi ("to be met, to meet (each other), to encounter"). Then the sentence is complete in the form Ni renkontiĝis en Londono.
Looking at the nouns, renkonto is a single encounter between two people or two groups, while renkontiĝo is vaguer, possibly referring to many people or sets of people in a process. That is why the latter is much more often used for the names of Esperanto events.