The term "incel" seems to have gradually become the default autonym for romantically unsuccessful people, having replaced other terms such as AFC, TFL permavirgin, blue-baller etc. This page will attempt to explain why this has happened.
In the 1970s, popular terms for those who couldn't get laid was simply "loser". In the late 1980, the term "love-shy" came into usage. During the early internet in the 1990s, the term AC, meaning average frustrated chump" was probably the most popular. From 2009, for about half a decade after, the term TFL, meaning true forced loner, was arguably the most popular on YouTube. The most popular equivalent in online forums was probably forever alone or FA in short. A look at google trends seems to suggest that "incel" has won out over all the other terms.
This is probably because TFL is not a true orthonym of the concept "involuntary celibacy". TFL is also a plesionym of the word "incel" because although they are almost synonymous, TFLists are more geared towards feeling lonely. The term "incel" on the other hand is a polyonym since it can be referred to by many different names, including "blue-baller", "permavirgin" etc.
Other autonyms to describe romantically unsuccessful people in the 2000s were "love-shy" and "involuntary celibate", although both have fallen out of use, probably because they are dionysms (consisting of two words). However, gradually from the mid-2010s onwards, the term "inceldom" has become the default autonym for the state of being romantically or sexually unsuccessful. One of the reasons that the term "incel" has replaced TFL and other synonyms is because it becomes a aptonym (word that aptly describes another thing) when you consider that the word "incel" sounds like "in cell". Its aptonymous nature derives from the fact that many incels do in fact feel trapped in loneliness, hence feeling like they are in fact in a cell.