There's other sides to this, but I do believe the main reason for male incels being incel is that they don't make any sort of move.

It doesn't really matter how hot or not the girl said incel pursues is because most guys have to initiate, furthermore I've even had numerous girls say that they don't really have the confidence/prefer the guy to make the move. I've even had girls say that there's been numerous times they've been interested in guy a (who may or may not be interested in them) but got with guy b, because guy b (who's not necessarily a bad option in himself, just not the initial one) made the move.

whilst some girls will make things 50/50, or put more effort on their part most of the times unless the guy makes a pass chances are nothing will happen.

even whilst some girls will flirt and make themselves sexually available to you (up for a date, give you their number, no problem being alone with you in their room, etc) the guy still has to take things to another level

if a guy is really bad at this, even if he is really good looking he'll have a hard time getting laid, and will end up being guy a from the scenario given a few paragraphs ago.

Fortunately I'm not incel but I have had my problems with women/getting laid and I was at my worse when I made zero moves. I've seen/read/heard the same thing happen with several other guys.

Heck right now I know a girl who's gone on 4 dates with a dude where nothing has happened and because he hasn't made a move for whatever reason, she's thinking that maybe he's not into her rather than thinking that she should take the initiative and now doesn't want to risk making the move herself because she scared of getting friendzoned.