I would say that there are subjective and objective reasons for the inceldom. Maybe we should talk about the different inceldoms (different kinds of inceldom according to the reasons).
Some subjective (controllable) reasons: shyness (when it's the only reason or one of the main reasons, i. e. when we're talking about sort of mentalcel (mentalcelness)), bad hygiene (also when it's the only reason or one of the main reasons), laziness (reason itself for many problems in the life, not only for inceldom; for example health problems, obesity, poverty, etc.)
Some subjective (uncontrollable or only theoretically controllable) reasons: shortness (when the person is too short for the present standards, i. e. a shortcel (shortcelness)), disability (when nobody around is willing to be with a disabled person (disabledcel) and when still there is no cure for this disability), poverty (in the areas where it's impossible to escape the poverty, i. e. poorcel (poorcelness)), old age (when one is too old, i. e. oldcel (oldcelness) according to the present standards), etc.
Objective reasons:: higher females' standards in most places around the world (about the looks, wealth, etc.), more choice for the females due to the developed internet connectivity in many places around the world, male:female ratio (in some places males outnumber females), i]polygamy[/i] (still in lot of countries around the world), lesbianism (female homosexuality), etc.