There is no main reason. Its a whole mixture of psychological, social, financial, political reasons.
There are Incels who cant land it because of their mental instability, non-neutotypical mind. Women have a nose for this, even tho certain things, such as sociopathy might actually attract a certain group of females. Yet, autism, aspergers, schizophrenia, ADHD, depression and bipolar behavior might give of Red Flags.
Economical reasons such as being a poorcel might actually play a huge factor, I even talked about it that a lot of above average guys cant land it because they have no status or secure jobs.
Which also plays in the financial problems: You rather eat and have a roof then have sex and even relationships are only happening these days if the man can provide - especially in western cultures.
Ultimately most will tell you that they are incels because of sub5 looks. Due to social media standards and hypergamy this major aspect is a reason for male loneliness.
Other social reasons of having never had sex is because one might be focused on other things. Like school, college, careery hobbies. A majority of redpillers are actually incels because they belive to have their prime in their 30s and sit out their teens and 20s for such things. The redpilled belief is that younger women are attracted to older men not because of looks and character but because these men are stable providers and it goes so far that an inceldom denier belives he eventually gets to choose.
Which - gets us to out last and most likely THE biggest reason: The Man NEVER chooses. You can be a rich gigachad but even so the Gigachad aint choosing. He just has all the attributes missing that lead to real inceldom. He has the quality and most women CHOOSE HIM.
